EDIT: Disclaimer: I only put here permanent modifiers, so no advisors, curia controler etc. Decisions aren't here, due to general level of difficulty of finding every one of them in order to properly compare hwich gives the biggest modifier. Too many countries have their own, unique decisions, so each of them would have to be checked. Sorry for that.

Base Attrition for Enemies: +3:

Dai Viet, Assam, Najd, Chickasaw, Shawnee, Afghan, Bosnia, Montenegro, Romania, Finland, Polotsk, Mapuche, Chachapoya, Armenia, Welsh, Charrua, Khiva, Berber, Divine, Central India or Siberia, Defensive (idea group), Quantity+Influence (policy).

Spoiler Venice, Genoa, Mamluks, Neapol, Hansa, Ragusa, Ryukyu, Mali, Javan nations, Navarra, Theodoro, Silesia, Danzig, Tarasca, Polotsk, Perm, Pommerania, Circassia, Bengal, Anatolia, Ireland, Suahili, Sumatra

So, what are the theoretical limits of in-game bonuses? I decided to count most of them, only the ones that can be done permanently (aka no temporary effects, things like piety etc.), and without decisions (it's quite difficult to find all fo them, especially these country-specific). Shortly: using only ideas (both national and idea groups), traditions, ambitions, full idea group bonuses, policies, religions and government typessArmy Tradition: permanent 100 as Brandenburg with defensive and innovative ideas and war college, or any country with said ideas and building and quantity+humanist and quality+espionage policies.Agressive expansion impact: -110%, as Hindu (with Shiva as PD) Rykyu with Influence ideas, Dip+eco policy and any 4 of these 5 policies: Dip+offensive, Espionage+expansion, influence+innovative, Aristocratic + Administrative, offensive+expansionAccepted culture treshold: -110% England, with humanist ideas' plutocratic + humanist and humanist + diplomatic policies, and enlightened despotism gov type, trading with silk.Advisors cost: - 98%, as colonial nation, with innovative ideas, influence+innovative, innovative+dip and innovative+plutocratic policies trading with tea.Possible advisors: +4, as Vijayanagar, Palatinate, Shawnee, Krakow, Ajuuraan, Pskov, Chimu, Chachapoya, Client State, West African country or Aymar with adm and economic ideas being reformed.Artillery combat ability: + 40% as Smolensk with Quality ideas and offensive+economy policyBlockade Efficiency: +133%, as Oman with maritime ideas and naval+maritime policyBuilding cost: -70% As Hindu (with Ganga as PD) Portugal, with economic ideas, aristocratic+expansion policy.Building power cost: -450% as Hausa with quantity ideas trading with tropical wood.Cavalry cost: -65% as Poland with Aristocratic and quantity ideas and expansion+defensive policy.Cavalry combat ability: +73%, as Poland with aristocratic and quality ideas and Aristocratic+espionage policyColonists: +5, as Spain, Muscovy, Norway, USA, Inca or Maya with Exploration, Expansion and Quantity+expansion policyCore-Creation cost: -68% as Hindu (with Shiva a PD) Ottomanns with administrative ideas (sidenote: tribal despotism gives another -10%, but I don't know if it's possible to change Ottos gov type to it. DDRJake only knows...)Culture conversion cost: -75%, as Muscovy, with religious ideas, influence+religious policy and being an administrative republic.Fort Defense: +143%, as Coptic France with defensive ideas and espionage+defensive, innovative+defensive, innovative+quantity, influence+defensive and influence+quantity policies trading with cofee.Diplomatic annexion cost: -70%, as Naxos with inluence ideas, Influence+administrative and plutocratic+influence policies.Diplomatic reputation: +14, as Austria, with diplomatic and influence ideas, being member of HRE (but not the emperor!) after the sixth reform and with 5 of these polices: innovative+diplomatic, aristocratic+exploartion, diplomatic+expansion, diplomatic+plutocratic, diplomatic+quality, humanist+influence, diplomatic+quantity or diplomatic+offensive trading with Ivory.Diplomatic relations: +7 as any Shoguante (practically onyl Japan) with diplomatic, expansion and influence ideas. (sidenote: Not sure if it's possible, but Provence has a potential to move to Japan, form this country after culture shift, and get +9)Diplomats: +8, as: Venice, Vijaynagara, Ayutthaya, Ragusa, Assam, Iroquis, Cherokee, Shawnee, Trebizod, Krakow, Bosnia, Karaman, Mapuche, Haussa, Circassia, Siddon, West African counries, with aristocratic, espionage and diplomatic ideas, embassy building, administrative+diplomatic, espionage+offensive and aristocratic+exploration polices.Discipline: +37,5% as Hindu (with shakti as PD) Japan with Quality and offensive ideas, being absolute monarchy and having economic+quality and quality+religious policy.Hostile core-creation cost: +260%, by Bohemia with aristocratic ideas, and diplomatic+defensive policy and embassy.Envoy travel time: -100%, anyone with influence ideas and naval+diplomatic, administrative+diplomatic and diplomatic+quality policies.Fabricate claim time: -53% as Papal state, Provence, Cyprus, Yaroslav, Polotsk or Naxos with influence ideas.Leaders without an upkeep: +5, as England, Hungary, Ryukyu, Croatia, Kanem Bornu, Bosnia, Inca or Colonial Nation with Aristocratic an Innovative ideas, war college and admirality.Galley combat ability: +75%, as Venice with naval ideas and maritime+quantity ideas.Global Colonial growth: +105, as Spain or Native country, with exploration, expansion and these policies: administrative+exploration; expansion+exploration; exploration+plutocratic; Humanist+exploration; quanity+expansion.Trade power abroad: +55%, as Venice, Genoa or Kurland with naval+trade and plutocratic+trade policies.Missionary strenght against heretics (note: it's purely against heretic, without general missionary stranght): +7%, as Finland or Bulgaria with religious ideas and religious-offensive policy.Missionary strenght against heretics (with global missionary stranght modifiers): +19% as Hindu (with Wishnu as PD) Najd with religious ideas, and policies: religious+espionage, religious+aristocratic, religious+diplomatic, religious+trade and religious+offensive.Global manpower modifier: +210% as Russia or Ming with Quantity and aristocratic ideas and quantity+exploration and revolutionary empire governmentMissionary strenght: +14%, as Hindu (with Wishnu as PD) Najd with religious ideas, and policies: religious+espionage, religious+aristocratic, religious+diplomatic, religious+trade.Global regiment cost: -35% as Poland, Armenia or Colonial Nation with quantity idea, and expansion+defensive policy trading with iron.Shipbuilding time: -40%, as Netherlands, Genoea, Oman, Raguse or Kurland with expansion ideas and maritime+economic and maritime+defensive policiesGlobal ship repair: +60%, as anyone, with maritime ideas and naval+administrative and naval+expansion policiesNational spy defense: + 14560% as Venice or Dahomey with espionage ideas and any 5 of these polcies: Innovative+espionage, espionage+administrative, defensive+administrative, religious+espionage, espionage+plutocratic, espionage+expansion or exploration+defensive trading with tobacco.Global tariffs: +110%, as Portugal with exploration ideas, and following policies: economic+exploration, influence+expansion, naval+expansion, exploration+religious and expansion+plutocratic trading with slaves.National tax modifier: +60% as Protestant or hindu (with Surya as PD) England with Economic ideas, plutocratic+economic and administrative+plutocratic policies, being an oligarchic republic.Goods produced nationally: +90%, as Pskov with Plutocratic and administrative ideas, and espionage+administrative, naval+economic, trade+quantity and religious+trade policies.Natioanl trade income modifier: +95%, as Hindu (with Surya as PD) Netherlands with trade ideas, being a dutch republic, with these polcies: maritime+offensive, trade+administrative, innovative+trade, offensive+tradeGlobal Trade power: +80%, as Aragon, Italy or Gujarat with Expansion and trade ideas and exploration+innovative and diplomatic+expansion policies and merchant republic with traders faction.National unrest: -12 as: Japan, Manchu, Lithuania, USA, The knights, Pskov, Client states, Ireland or Natives, plutocratic and humanist ideas and 5 of these policies: espionage+defensive; dfensive+religious; defensive+economic; humanist+offensive; humanist+espionage; plutocratic+influence trading with fish.Heavy ship combat ability: +30% as England, Spain or Pomerania with naval ideas and innovative+maritime policyIdea cost: -40% as: Milan, Ming, Mughals, Korea, Savoy, Granada, Cheerokee, Shawnee, Trebizod, Javan nations, Navarra, Aztecs, Polotsk, Inca, Wales, Bahmanis, Hamburg, Colonial Nations, Daimyos, Mesoamerican nations, Tibet, Aymar, Natives; while being protestant, having humanist ideas and humanist+trade policy.Improve relations: +130%, as Yaroslav with diplomatic ideas and naval+diplomatic, diplomatic+plutocratic and humanist+influence policies.Infantry combat ability: +50%, as Sweden or Prussia with quality ideas and innovative+quality policyInterest per annum: -54, as Hindu (with Ganga as PD) Austria, Hansa, Tuscany, Gotland or Hamburg with economic and administrative ideas and innovative+trade policy.Reduce inflation cost: -43% as Aragon with economic ideas.Yearly inflation reduction: -0,25 as Netherlands, Colonial nations or Andeans, with economic ideas and Tax assessor building.Land Attrition: -95% as Kongo or Horde (with horde ideas, not gov type), defensive ideas and quality+exploration and humanist+defensive ideasLand forcelimit modifier: +198%, as Muscovy or Daimyo, with quantity and offensive, and diplomatic+offensive policy and economic+quantity and grain depot trading with grain.Land maintenance modifier: -95% as Scotland, with defensive, economic and quantity ideas, and administrative+quantity policy and grain depot with merchan republic government with aristocratic faction trading with salt.Morale of armies: +90%, as Shiite France or Brandenburg, with defensive and plutocratic ideas, quality+religious and religious+quantity plicies, and noble republic, dictatorship, merchant republic with aristocrats faction or revolutionary republic governments being a defender of the faith.Land leader fire: +3, as Bohemia, Smolensk or Rajput with offensive ideas and expansion+quality policies.Land leader maneuver: +4, as Shawnee with defensive ideas and quantity+exploration policyLand leader shock: +4, as Sweden, Scotland, Manchu, Lithuania, Walachia, Ajuuraan, Air, Arawak, Mapuche, Charra, Hordes, Arabs or Tibet with Offensive and Aristocratic ideas and humanist+quantity policy.Naval leader fire: +4, as Dutch with naval ideas and naval+innovative policyNaval leader maneuver: +4 as Oman, Mogadishu or Suahili with maritime ideas and innovative+maritime policy.Naval leader shock: +3, as anyone, with naval ideas and naval+religious policyLeader siege: +2, as Nepal with innovative+offensive policyLight ship cost: -35% as Hansa, norway, Oman, Novgorod, Mogadishu, Navarra, Sidon, Gujarat or Suahili with Maritime+plutocratic policyLight ship combat ability: +35%, as Norway with maritime+plutocratic policyManpower recovery speed: +75%, as Sweden, Ottomans, Manchu, Iroquis, Georgia, Wallachia, Karaman, Romania, Hordes, Anatolia with Quantity ideas, constitutional monarchy and administrative+quantity and quality+exploration policies trading with cocoa.Mercenary maintenance: -78% as Sweden, Burgundy, Wallachia, Smolensk, or Daimyo with administrative ideas trading with cloths.Mercenary cost: -115%, as Huron with innovative and administrative ideas and espionage+quantity policy.Merchants steering towards inland: + 95, as Air, Songhai, Persia, Hause with trade ideas, defensive+trade and naval+influence policies, being a merchant republicNaval attrition: -85%, as Venice with quality ideas and humanist+maritime policy.Naval Forcelimit modifier: +168%, as Netherlands, Denmark, Knights with Quantity, maritime and exploration ideas, and maritime+expansion policy trading with naval supplies.Naval maintenance modifier: -63%, as Genoa with naval ideas and maritime+economic policy being a merchant republic with traders faction.Naval morale: +55%, as Ibadi England, Norway, Genoa, Malaya with quality and naval ideas being a defender of the faith.Available mercenaries: +225%, as Burgundy, Gotland or Vijayanagara with administrative and plutocratic ideas, and expionage+quantity policy.Privateers efficiency: +53% as Naxos or knights wtih espionageProduction efficiency: +85% as Punjab with economic ideas, Aristocratic+innovative, humanist+aristocratic, naval+economic and trade+economic policies beind an administrative monarchyColonial range: +123% as Norway with exploration ideas and exploration+plutocratic and naval+exploration policiesRebel support efficiency: +150%, as ireland with espionage ideas and espionage+econimic and espionage+humanist policiesRecovery army morale speed: +40%, as Pskov with quality ideas, and quality+administrative, religious+quantity, expansion+defensive and humanist+defensive policiesRecovery navy morale speed: +35%, as Naxos or Neapol, with quality ideas and religious+maritime, naval+espionage, naval+innovative, naval+religious and humanist+naval policiesBetter relations over time: +143%, as hindu (with Ganesha as PD) Italy or Iroquis with humanist ideas and humanist+diplomatic and administrative+aristocratic policiesReligious unity: +175%, as Jaunpur, Bengal, Gujarat or indian sultanate, with humanist ideas and these policies: defensive+religious, religious+diplomatic, humanist+aristocratic, exploration+religious and religious+offensive.Ship durability: +25%, as England, Denamrk or Korea, with naval ideas and maritime+offensive and humanist+naval policies.Siege ability: +45%, as Hindu (with Shakti) Netherlands or Smolensk, with offensive ideas and innovative+offensive policies.Spy offense: +85%, as Trebizod or Aztecs with espionage ideas and innovative+espionage, offensive+expansion and espionage+offensive policies trading with spices.Stability cost modifier: -145%, as orthodox wales with religious ideas and defensive+economic, defensive+administrative, humanist+plutocratic and influeance+quality polcies, being bureaucratic despotism and fine arts academy trading with wine.Technology cost: -37%, as Muscovy, France, Bohemia, Manchu, Cherokee or Lan Xang with plutocratic and innovative ideas, being the member of HRE after the first reform, with at least 25 members of HRE and with university.Tolerance of heathens: +9: Buddist or Confucian Ottomans, Mughals, Granada, Bahmanias, Siddon, Jaunpur, Bengal, Gujarat, idian sultanate, wit humanist ideas, humanist+exploration policy and dictatorship government.Tolerance of Heretics: +11: Reformed, Buddist or Confucian Lithuania, with humanist ideas, religious+plutocratic policy and revolutionary republic government.Tolerance of true faith: +8. Coptic Byzantium, being a theocracy, with religious ideas.Trade efficiency: +90%, Many countries, with trade ideas, being a constitutional republic, with trade+quality policy and 4 of these 5 policies: naval+administrative, trade+economic, trade+expansion, naval+trade, offensive+trade.Trade range: +78%, Ryukyu with trade ideas and naval+exploration policies.Trade steering: +103%, Oman with trade ideas, Trade+expansion and defensive+trade policies.Unjustified demands: -85% as Creek or Siberians with influence ideas, being a despotic monarchy.Subject forcelimit contribution: +175%, as Bosnia with Influence, influence+offensive, influence+economic and influence+aristocratic policies.Income from vassals: +218%, by Persia, Timurids, Ayyuthaya, Kanem Bornu, Bosnia, Ashanti, Mesoamericans with influence and irostocratic ideas, offensive+administrative, influence+economic, influence+expansion and influence+quality policies being an iqta.War exhaustion: -0.125: siberian with innovative being the Defender of The faith.Cost of reducing war exhaustion: -116%, as Iroquis or Shawnee, with diplomatic and aristocratic ideas and influence+defensive ideas trading with sugar.Years of nationalism: -25 as Timurids, Tarasca, Ashantim, Horde or Ruthenia with humanist ideas, humanist+offensive policy and constitional monarchy or administrative republic government.Yearly republican tradition: +0,9 as Novgorod, USA, Hamburg, Colonial Nation or italy with plutocratic+economic and diplomatic+aristocratic.Ok, that's all I did, for sure there are a few more, but I don't have the nerve for some of them/some are too general and low (like legitimacy, which is at max +2,2 but for a lot of countries, with only other modifiers being 2 polcies and being the member of HRE after the 3rd reform). Some of these limits are harder to get that WC, also I don't know the caps. But this list can be used for testing (I.e. rebel support efficiency to test how exactly does it work). Some I like pretty much since they require very strange combination (Shiite France, anyone? Maybe Ibadi England? So maybe Coptic, theocratic Bizantium?). on a side note: why does Byzantium has higher tolerance of true faith than Papal state?!Ok, I know it's very long, but I hope it's worth reading, or at least checking some statistics. Anyhow, I'm open to You for pointing any mistakes I made.EDITS: Mistakes pointed out in comments are put here ASAP.