Soviet T-34 Tanks
Soviet t-34 tanks are designed to swarm the enemy, though not particularly armoured they have decent weaponry and are quite cheap. They are the primary tank of the soviets and seeing a company of these will make even a panther tank think twice about engaging.
T-34 (mid/late war)
The backbone of a soviet tank platoon. The T-34 is fast, not particularly well armoured but also very cheap. Great for flanking the enemy or engaging infantry, these fast tanks are best used in numbers. Like all soviet forces taking losses is just a part of winning, you just need more tanks than the enemy can kill!
If you are playing mid war you may opt for the up armoured version. it loses its fast tank ability but gains an additional point of armour on the front and sides around. Against very highly armoured tanks the faster version is sometimes the better choice as even with the stronger main gun you will still need side armour shots to destory them.
T-34/57 (mid war)
If you need some extra firepower to take out heavier armour the t-34/57 can give you that extra punch. It is still a fast tank which makes it useful for flanking.
T-34/85 (late war)
This is the late war replacement of the T-34/57. It has better anti tank firepower and slightly better front armour, but it loses its fast tank ability. Given that in the late war you start seeing some very heavily armed and armoured tanks, this is a good trade off.