Here's how it goes:
Spies think every pyro is OP
Every pyro thinks soldiers, demomen, and heavies are OP
Every soldier thinks snipers and scouts are OP
Every sniper thinks spies are OP
Every scout thinks heavies, pyros, and engineers are OP
Every engineer thinks medics and spies are OP
Every class thinks engineers are OP.
The reason TF2 is CLASS-BASED is because every class has a very small advantage over another.