Ahhh, nostalgia.
Fallout saga, Monkey Island II & III and X-Com saga are the games that get reinstalled onto my comp about once every 2-3 years.
T.A. was a great game too. So customizable, yet so few people ever made new chapters for it. Just maps and units. Pity. Supreme Commander is a good replacement at least.
I'm really amazed that the X-Com series just died like it did. It had so much potential. I blame it on that crappy FPS version that they made. Can't remember the name of it.
The game that I LOVE to replay over and over is Star Control 2. The graphics are still pretty nice by today's standards, as they were quite cartoony. The music was great, whether on PC speaker or Soundblaster (and the opensource version has new remixed music that makes great disco tracks).
But the best thing was that it was non-linear. You could do the quests in nearly any order.
That, and some of the one-liners in it, like the Syreen hitting on you, or the Umgah insisting you'd be happier if only you had some tentacles and a few extra eyes.