The Set I'm Discussing Registeel @ Leftovers

Ability: Clear Body

EVs: 252 HP / 4 Def / 252 SpD

Careful Nature

- Stealth Rock

- Thunder Wave

- Seismic Toss

- Earthquake

Registeel Unranked -> C or B-.[Do keep in mind that I feel educated with Registeel, but this is still subjective opinions based on objective data. You can draw your own conclusion on the pokemon]Reasoning:Pros:I personally believe that Registeel has a very unique niche and is one of the greatest single mon counters to the popular Dragmag strategy when it is on rain teams. While specs Magnezone can deal considerable damage to it, Registeel still overpowers it with Earthquake, and can absorb specs tbolts with its monstrous 438 spdef stat. Other sets with air balloon/magnet rise to negate the ground type weakness are outdamaged by seismic toss. In addition, it's ability to use thunder wave and stealth rocks adds extremely versatile utility and supports your other pokemon for the late game. Thunder wave is a great way to counter Dragonite - one of the premier partners with Magnezone, and even a specs surf in the rain from Latios (arguably the best pokemon in the metagame, and an S tier mon) does not 2hko Registeel. Registeel can paralyze Kyurem-Black, and can also tank two to three hits, even if it gets outdamaged. Registeel also does surprisingly well against Jirachi in the sense that it can paralyze it, completely wall it, soak up special attacks even on calm mind variants, and earthquake for substantial damage. It also has potentially a winning matchup vs Thund-T, in the sense that it can weaken it with rocks and tank a few attacks (not on focus blast variants, but those are generally less common than grass knot/double boost nowadays from my experience). Although unreliable, Registeel can be a check to Tentacruel, as immunity to toxic and earthquake pose a threat, and thunder waving and setting up rocks overwhelm the pokemon. What I mean by this is if it is trying to protect stall with rain dish and black sludge recovery, paralyzing the pokemon and putting up rocks forces it to hit through paralysis to keep tempo and rapid spin the rocks away, buying Registeel time to whittle it down.. Registeel is a reliable counter to some of the most threatening dragon types in the tier, with the added bonus of being able to check Magnezone - the premier counter to steel types in the tier. It also has surprisingly good matchups against other threatening pokemon.Cons:There are a few things holding Registeel back from going any higher than the places I have recommended. For starters, it is really only good at one thing, which is countering dragmag. While dragmag is a large threat in the metagame, Registeel really does not pull its weight against standard rain teams, and especially sand teams - aside from the occasional thunder wave on a threat. Garchomp is a major counter to this pokemon, as is Excadrill, and both are huge pokemon in this tier. Both Gliscor and Landorus are immune to both earthquake and Thunder Wave, and check Registeel with little to no problem. There is also the opportunity cost of running this pokemon, as Ferrothorn is another steel type pokemon that is usable in a lot more circumstances - even if it is trapped and loses to Magnezone often. Additionally, unlike a lot of the top tier threats, this pokemon really only has one good set (in my opinion), which makes it telegraphed and predictable. With all of this being said, I still believe that Registeel deserves its spot in the metagame (and I would love to see more people using it).Synergies:Ice Beam Politoed (particularly defensive variants) and Specs Keldeo: I personally think that this is by far the best synergy with Registeel, as setting up the rain and providing ice type coverage are two things that heavily benefit Registeel. The rain allows Registeel to easily tank 5 hidden power fires from specs Magnezone, as well as other fire type moves from prevalent threats such as Fire Punch Dragonite. However, Ice Beam is where Politoed is extremely helpful to Registeel. Aside from Swords Dance Yache Berry Garchomp, ice beam obliterates every ground/dragon or ground/flying pokemon in the meta, nearly KOing/KOing with a roll on all of them and tanking a hit as well. However, I would personally say that this makes the core particularly weak to SD chomp and Kyurem Black, as you have no reliable way to kill it. This is where specs Keldeo comes in. Keldeo outspeeds Garchomp, and Hydro Pump kills even when the rain is not up. Additionally, you can KO with Icy Wind if the Yache is popped by Politoed. Specs Keldeo also KOs Kyurem with one Secret Sword, and is already one of the premier breakers of the tier. The biggest counter to specs Keldeo is Latios, but Registeel is able to beautifully handle that pokemon. Therefore, the three of these pokemon make a powerful rain trio that compliment each other nicely.Donphan - I will never understand how this thing has ice shard. Although Registeel's main function is to defend against Dragmag, it still falls short vs Garchomp. However, Donphan excels vs Garchomp, and Ice Shard is a great option against Garchomp and every other 4x weak to ice pokemon listed in the previous synergies section. Donphan is stupendous at dealing with the main sand pokemon, as it walls even banded Tyranitar and obliterates Excadrill. Donphan is pretty susceptible to ice type damage, but Registeel handles that greatly. Additionally, Latios is a pokemon that Donphan can barely touch even with SE Ice Shard, and gets KOed on a roll even from scarf sets, but Registeel is a strong counter to Latios, which allows the two to compliment each other.Conclusion:This pokemon is UU for a reason. It will never be a staple of the Gen 5 OU meta. With that being said, I believe that with strong teambuilding, Registeel shuts down the gameplan of a huge portion of the metagame, in Dragmag. Being a steel type that can counter Magnezone is unheard of in the meta (the two main steel types in Jirachi and Ferrothorn are walled and easily killed by Zone, but Registeel can beat it and serves other purposes). I therefore believe that it deserves a spot on the viability rankings, as it is indeed a viable pokemon.LMK if I've made any mistakes or if you have an opinion on Registeel :).