eh it somewhat depends, as of late I have gone engie alot because either our engie's were fucking retards about placement of stuff or we didn't have any so the other team went scout crazy on grabbing the intel >:|
although when i am free to choose, I generally go pyro or demoman, pyro destroys spies (I fucking hate spies to death, so setting the whole damn map on fire into the depths of hell is mass amounts of fun for me) plus the airblast is the most fun I have on the game, period. Blowing a crit rocket back at the other team for a triple kill? massive lulz, plus the fact of airblasting dumbasses, ESPECIALLY heavies off the edge, is just too much fun.
demoman is fun in the aspect of destroying opposing snipers high up or destroying annoying fast/jumper demomen who are purely melee or sky jumpers