Pyro is my faviourite.
But I'm easially best at the engineer. Defending with the engineer is...Ok. But if you go offensive people go fucking crazy. In 2fort grab a soilder, tell him to stick with you, stick down a sentry, teleporter and despenser in the enemy base at the entrence to the sewer. Then, if anyone trys to pick a fight with your sentry, show him your shotgun, or, for major humiliation, your wrench.
Defending in 2fort can also be fun in this way. No one expects a engineer to kill a heavy, but if there clever enough with that wrench and shotgun, an engineer can be fucking ruthless.