Chinese Tree

Grinding Chinese heavy tank tree line first is preferable to medium or light line. Going up from the Type 58, the grind is slightly easier, due to medium line to getting light tank matchmaking. However, the light tanks are much more fun to play. Not to mention WZ-132 can be used in CW if it comes down to it.

Researching 113 heavy tank gives you WZ-120's 122mm gun for free.

WZ-111 Models 1-4 gets T-34-2's top engine for free stock, and it also shares top engine with WZ-120.

110 shares its 100mm 62-100T with the WZ-120.

IS-2 gets T-34-2's first engine upgrade for free. IS-2 also shares the 122mm 37-122JT with the T-34-2.

USSR

Going up the IS-7 heavy line first is preferable to going up IS-4 first.

If you go up IS-4 line first, you will not have the 122m BL-9 on the ST-I. Grinding for it on the ST-I without free XP is horrible.

IS's engine upgrade can be used in the IS-3, ISU-152, and Object 704.

SU-122-54 shares the same engine with the T-54. It may be better to get it on the SU-122-54 first, as the TD does not depend on speed as much as the T-54.

Grinding SU-101 can get you the top engine for the T-44. Or you can go straight for the SU-122-54, and get the first engine upgrade on the T-44 for free.

France

AMX 50 120 shares the same engine with AMX 50 Foch.

TD and heavy line share lots of engine upgrades.

Germany

Most lines share engines. Unfortunately there isn't 'free' modules to be found in high tiers. (Leopard 1 has the exact same engine as the Leopard PTA, but the Leopard 1's engine has an A. So it doesn't get it. China bias plz)

UK

Going up the heavy tree first unlocks the guns and engines for the medium trees immediately. They are not free, but it's better than grinding the Centurion Mk.I with a 17pdr and stock engine. TD and arty tree gain some engines from the heavy tree as well.

USA.

Grinding up turreted USA TD trees unlock a lot of engines for the US mediums.