Right Behind You. This theme has been known to induce panic and horror if played in-game. It's chilling, quick-paced, and gets the message across that the class it represents (Spy) is not to be underestimated.
Drunken Pipebomb for the black Scottish cyclops. It deserves credit for actually making bagpipes sound awesome.
Scout's theme, Faster Than a Speeding Bullet, a fast-paced combination of a catchy piano riff and an almost inhumanly fast bongo solo. In can show you he's quite powerful, if you know how to use him.