[Bwaaaa]

Soldier hands down. He has the mobility by rocket jumping, the highest damage output due to juggling things with your rockets till they either die or quit the game out of rage and just the most fearsome presence.

You can avoid a Heavy easily but a good Soldier will rocket jump and rain death onto you and your little Medic friend with no way to escape.
He just is the one class that does everything well with crazy roaming abilities, making it a Scout on steroids.