Hard to remember... buuuut...
ToME was and is a great one. Though I usually take brakes of months. Finding everytime new updates and amusement.
Liero would be another all-timer. It's not as if I play it often and so; but now or then I pick it up and start to play it again.
From Snes I can remember the Rock and Roll Racing and some shoot-'em-ups that are great.
For multiplayer AvP is a typical in my 2-men-LAN's with a friend. And cooperative Serious Sam Second Encounter became also a must.
Majesty also comes now and then to my hands and I play it till I come to the ultra-difficult missions, which usually make me delete it for some months.
Jedi Knight II: Outcast was a great game. But I don't really know if I could play it all over again. But, hell, was it good.
Although I usually prefer to watch the games. Look my friends playing them and "helping" from the outside... or just having a drink/smoke/snack while he/she has fun with the game. Doom 3 was funny in that way, Half Life 2... also some other X-Box/PS2 games I really enjoyed watching them. Same with GTA San Andreas and some others new games.
I just like to observe, an overwhelming joy.
C.