I've played a bit, though I'm currently distracted with watching One Piece. :p So far my favorite to play has been Pyro. I've done the best with him, having earned the most of his class achievements, had most kills, etc. Next fave is Medic, since I can get credit for kills and such without actually having to attack anybody. Just follow around a Pyro or Heavy and get credit for assists while they mow down everyone around us.
With life-long pal, the Friendly Yeti.