Scout: something like heavy or engineer should do

Soldier: Pyro is a direct counter for soldier, being able to redirect his rockets, but again, heavy, engineer, spy or sniper should do due to his low mobility, even when he's rocket jumper you can just pierce him with heavy

Pyro: just stay away from his range, otherwise you're most likely to lose

Demoman: with scout you should be able to dodge his bombs, spy could kill him if he's just walking.

Heavy: something like sniper, spy or soldier could do, but if you're skilled enough you could strafe him with scout

Engineer: spy is the direct counter for engineer, however if you dont wanna play spy another thing you can do is use soldier or heavy to shoot out his sentries from afar.

Medic: just wait for his ubercharge to end

Sniper: in close range you can just pick something like pyro, or if he's completely aiming you can use spy, and if he uses razorback you can try and 1v1 him with the revolver

Spy: once again pyro is the direct counter of spy, because you can just burn him alive, however you can also use sniper with jarate to make his cloak useless.