Pretty much all of them, though my top classes are Pyro and Heavy. I also enjoy all the others (aside from Scout and Soldier, which just don't suit me and actively tend to piss me off when people overuse them).
I'd normally have said Heavy outright, but the problem is that the people who play as Medics don't understand that Heavy+Medic= beginning of large-scale offensive push. You work up around the fringes, kill enough enemies as a duo, then Kritz/Uber and charge their flag as all their fire starts coming your way. Even if you don't get them all, your teamates do, if your teammates have any brains.