Well, IS/KV-3 aren't as powerful anymore since Tiger I got it's rld/dpm buff. KV-1S and KV-2 are just derps and the stats are probably that way because these tanks are used a lot in Tank Companies for Tier 6, and KV-1 just because it also has a derpy gun an Tier 5.
Didn't really play the whole Medium line, I plonked into it via KV-1S and KV-13, and the latter was actually already quite fun. T-44 needs Wet Ammorack and you'll be fine, it's decent, not OP. T-54 is a beast, not more to say about it. Since I really started getting into the "how-to-drive-t54", I seldomly have games with less than 2500 dmg.
Heavy-line... IS-4 is tanky, 2500hp and good angles, but imo worse than IS-7, which just eats shells like no tomorrow (best game with over 17k pot-dmg received). Russian tanks in general are good for starters, as they are easy. Germans are the hardest imo, but very rewarding in the end. Muricans... inbetween. Good turrets, but at tier 8 underwhelming guns, so I usually go for Premium-Ammo. M103 though is one hell of a tank.