The ranges for the cannons are a joke for the sake of game balance, as a 17lb gun had an effective range of 3000m, but they're all pretty balanced. It pretty much comes down to preference, though the 90mm has a place at the top.

Yeah, 90mm (3 inch for US) will do for most targets, but you may need a bigger stick when facing tanks like the IS-7 or Mau5, or generally any large post war tank.

20pdr on the Centurion Mk. VII has a really really fast ROF, really good accuracy, and also pretty good penetration. Max range can usually let it deal with most static AT guns too (I think it's HE range is 150). When you don't need a gun specifically to kill one of the super armored beasts it's probably the best. Still I've sent an APCR round through the E 100 turret front before and it can easily pen it from the side so it's not like it's complete hopeless against them either.

They can also spawn in from the random medium button for 500 too. When I was playing the Korean map yesterday first 4 tanks I spawned in were all Centurions.

Against the really tough nuts, shot placement becomes key. You can have the biggest stick in the world, but it'll just bounce off of some armour. Tracks, Main Gun, and Engine are my main targets, and them it turns into a mop-up op with infantry and TNT.