1) Fallout - Rather than be redundant by listing things about the game itself, I'll just say that NMA is the first internet forum I have registered at, read often, and actually post on. That's how much I like Fallout.
2) Fallout 2 - I have my criticisms, but it remains the game I replay more than any other (although if importing FO into 2's engine sparks a bunch of mod-building, that may change in the future).
3) Deus Ex - You know, adding some rpg-like elements (skill points, inventory management, a few meaningful choices) to a fps really made it enjoyable. And it didn't hurt that there really were very different ways of completing a lot of the missions.
4) Arcanum - I love it. And for the record, I love the Car Arcanum mod even better than the unmodded game. Call me crazy.
5) Vampire: The Masquerade - Bloodlines - Allthough it has the clunkiest title of this whole list, I like it.
6) Silent Hill 1 and 3 - Nothing, I mean nothing, has these games beat for mood and fright. Spirit walking in Prey, spirit walking in that Legends of Cain sequel, none of them is as good as the first time the abandoned school wobbles around you and rematerializes as a blood-stained, grated, mewling baby-ghost infested horror-show nightmare place-you-just-want-to-get-out-of.
7) Descent 1 and 2 and 3 - For the Mac only, I guess. But seriously the first and only game series I have played that made FULL use of a 3d environment. I mean a maze of tunnels that really was the full range of up and down and everywhich way. it reminded me of Ender's Game: "The door is down!"
8) Dooms (the original batch, NOT Doom 3) - My college buddies and I switched lots of the monsters' sounds with Sesame Street music and voices - my first experience of modding!
9) Galaga - Yes, the arcade game. And to this day I suck at it, but that whole getting two ships linked together remains one of the coolest things ever in any game.
10) Planescape: Torment - I loved it, but it gets bumped way down here on the list because, well, it lacks replayability for me. Maybe I had to play it 2 or 3 times to catch all the cool speeches, but, I don't re-read books very often either. Maybe every 5-10 years I can reopen Watership Down or Dune. PT will live on the shelf, much honored but not replayed as often as the others on this list.
Misteryo
*Edit*
And minesweeper. I rock at that game!