Golurk

For me, he ties with Pyro. I basically enjoy playing as those two classes and nothing else.

Or do you mean from an actual effectiveness standpoint?

More from a "fun to play as" perspective. I mean, you've got someone whose whole purpose is to blow stuff up. And his weapons more than accomplish that. Not to mention sticky jumping, the most physically possible thing ever.

It's also really fun to grab a sword and shield and just run at people. Or even better, do it with the Pain Train.