My top five:

1. Atari Star Wars
2. Atari Tempest
3. Atari Asteroids (maybe one of the later variants, but the original gets cool points)
4. Williams Joust
5. Dance Dance Revolution (had to put something vaguely modern in the mix...)

Of course, this is leaving out so many other good games, like Tron, Sinistar, or even Moon Patrol, but I've always been partial to the Atari vector games. (And, it's such a treat to find one that's still working. Those color vector screens didn't age very well.)