Galaga holds a special place in my heart. It's one of those games that I can always come back to, because it's a constant test of my dexterity and attention. Sure, I memorize the bonus levels, but it's not as stressful for me as Joust or Bubble Bobble have become (with Bubble Bobble, I'll often just outright quit if I die before level 30).
There's a similar old game that I used to play which I used to love the crap out of, IIRC it's called Galaxian, but I'd need to play it to be sure. What I liked about it was that it had a destructible environment, like space invaders, but far more detailed. Maybe I'll try to hunt down a ROM and see if I'm remembering the right name for the right game.