It’s stock gun is really pathetic.
Basically the gun upgrade order is: 122 mm U-11, then the 107 ZiS-6 (or maybe the 152mm M-10)
The 122 doesn’t require the turret upgrade, but only has HE ammo, it’s a decent gun, not great, but it will let you actually do something besides bounce shells off of medium tanks of the same tier.
The two guns that require the turret upgrade, both are fun, I like the accuracy of the ZiS-6, along with its faster reload times. But the other one, the ‘derp’ gun, will let you one shot a lot of tanks, just don’t miss, since it takes forever to load.
I really enjoy my KV, in games were I’m top tier, I’ve gotten 8 kills in one battle, and can routinely get lots of kills, while lots and lots of shells just bounce off of you. And even in games where my KV is mid or low tier, I can still contribute since I have good range and damage. And once you get the engine and track upgrades, its not that slow. I also highly recommend buying the trained crew, I started at 75% and I’m now at 93% crew rating, which really helps.
One last tip I’d give, is find all the arty blind spots on the maps, that’s where the KV needs to hang out. I find it’s pretty good at city brawling, just alright in the forest, but only with a spotter, and try not to go by yourself, the turret traversal is pathetic, so a single medium tank can ruin your day if caught in the open.