Based on the way you were talking it sounds like you wanted to do a poll...but theres no poll here. Oh well.
Anyways, I always think of either the Heavy, Spy, or Engineer. Probably because I play those three classes almost exclusively. But honestly, what are the characters that make this game stand out from all the other shooters ever made? A character who usually doesn't deal with direct combat, a character who requires massive amounts of skill to play well, and a massive Russian with a penchant for Sandviches?
Poster children all.