Ambassador: Crit headshotting lame Engineers turtling behind their equipment with a long line-of-sight outside Sentrygun range. When running away, getting a crit headshot on the persuer is always worth a few giggles.
Engineer Pistol: Long range, relatively accurate, and 200 shots. Sometimes I'll use it instead of my shotgun for shorter range.
Engineer Sentry Gun, Level 1: 2 and 3 are great, but there's nothing like that first surprise kill of the soldier with a fresh level 1, when he's getting poked to death spinning around like a whirling dirvish looking for the source of his pain.
Flare gun: When people run away from me, at just about the range where they get smug that I can't hurt them, the suddenly burst into flames. Shotty range isn't that much better than flamethrower range, and can't catch people on fire. I just avoid maps with water.
Scattergun: I'm coming to like this weapon.
Bat: Nothing more humiliating than being boinked to death.
Blutsauger: DEATH FROM ABOVE! DEATH FROM ABOVE! (Especially on Warpath) And just in general, being able to run away and survive almost anything. And Blutwars between Medics
Ubersaw: Rattle SHING! 25% uber. Booyah.
Natasha: Sounds like ack-ack gun. Good sound. Makes scouts slower. Not get sandvich this time.
Rocket Launcher: Sparkle rockets are pretty.