The US mediums are a blast and you can't go wrong with them, the T20 while being a bit rough at first (sorely needs the 90mm gun) is pretty nice as a brawler/mobile sniper and if they didn't change it much it still makes a nice buck. The Pershing and Patton after it are also a blast to play being very versatile and able to use the terrain to the fullest, but still, slightly bad until you upgrade them (the Patton especially, needing about 100k xp to become awesome).
Soviet heavies are pretty good to, being very forgiving and straightforward to learn and use. If you're enjoying the KV-1S I'd suggest going for the IS, which is more of the same, but slightly better armor and RoF, the IS-3 after it is one of the best T8 heavies having nice armor, gun and mobility. IS-8 is once again more of the same but with an even better gun and mobility, the armor might be lacking in some cases but the sides are shell eating as always.
As for the TD line (not sure if you're on the JPanzer 4 or have you already gotten the JPanther?) they're all very nice tanks, the JPanzer isn't the best out there but it does it's job decently enough, the JPanther on the other hand is one of the best mobile snipers out there, you can either go for moneymaking with the 88/L71 or damage output with the 105mm guns. Afterwards you have the Ferdi which is slow, big and heavily armoured, the gun will be a struggle until you get the 128mm which is pure awesome. The other one, JPanther 2 I haven't played but from what I hear it's like the JPanther but it gets better guns and some extra armour.
French light/medium line is pretty nice too, I personally dropped the Lorraine after it was moved to T9 (didn't like how squishy it felt now) but all the AMX's leading up to it were a blast to play, especially once you get used to the playstyle of a vulture and taking advantage of openings and weakend foes (this will also translate well into the other medium tank lines).
As for the Brits, I haven't got past tier five yet but the Cromwell/Comet seem similar to the E8 both in terms of stats and playstyle while the Centurions are pretty awesome snipers with great guns, but somewhat poor mobility and survivability.