Goldeneye on the 64. Dang if that game ate a chunk of my life. One of the best games of all time.
Diablo 2 ate a vast chunk too... Where I discovered multiplayer, and discovered that going to bed at 8am has side effects.
Rollercoaster tychoon.
Deus Ex.
Zelda 1 and zelda 3. Both amazing games. First game I played was 1, favourite zelda game is 3.
Baldur's gate 1+2+expacs. Loved those games, seriously good fun, and good story to 'em too. Also, nothing as fun as having an epic magic battle with a demi-lich. Or going back to baldur's gate 1 after completing throne of baal and playing through again.