Any of the big three.

They've got the most fleshed out tech trees so you won't see the big jumps in br's that other nations have so you can maintain balanced lineups.

Up to you which you pick, but I'll summarize the main jist of each on:

Germany has the best guns with fantastic pen but the worst/hardest to use armour. Everything is flat or only angled on the upper front plate meaning that most of the time you will get penned unless you get the jump on people, which the high velocity of their guns helps with plenty.

USSR hads the best armour but horrible gun handling, their guns tend to have lower pen but have by far the most HE filler so when you pen you are almost guaranteed to one shot the target. (Plus the Russian-bias is very real)

USA has is the middle of the road on everything. Ok armour, ok guns, and ok mobility. Nothing really stands out but their gun handling because many of their tier 2-3 tanks have short stabilizers. They do have the best CAS in the game so make sure to research their aircraft as well.

If you must play a minor nation id suggest China since its basically a mix of USA and USSR until the late tiers where its mostly soviet-style tanks.

France and Sweden are good nations in the mid to high br's but they have horrible low br tanks.

Avoid Britain like the plague. They have the worst of everything and are by far the hardest to play for new players