One of the best (And worst, depending on your class) things in the game: Colliding with enemies. Sucks for Spies, because you shimmer, but it's great for the Scout. Try jumping on a Heavy's head and batting them to death. It's awesomazing.
My best class would probably be the Spy, and I know I will be flamed, but I like the sneakiness of disguising & waiting for enemies to divide up a bit, then kill groups of them.