Make that spy and you would be good. You never kill off or even weaken the heavies and you always get killed by scouts and pyros.
For me it is soldier. The ability to easily take out any class but to still have a challenge is nice.
It's becuase you're slow and terrible just like the soldier.
Mine is scout because he's fast and if you're good (like me), and have good accuracy, you can kill anything and everything.