There's a bunch of those games that I love infinitely and always come back to, usually like at least once in two years or so:
TES: Morrowind, not much explanation needed. I've played it so much that I actually might be unable to replay it anymore. Then again I feel like doing a very final run in which I'd collect and store somewhere every collectible item (including spoons etc) in the game. Then I realize how stupid it sounds and change my mind.
Doom, I have it installed even now. I like to play some random wad or fuck around in level editor. Muh true love since 2000 or so when I first played it.
Fallout Tactics, a lot of people hate on it but there's something in this game that just makes me come back. I might've finished it more times than all other Fallout games in total. I replay it once a year on average.
Commandos 2, possibly my favorite game ever. Simply perfection.
Baldurs Gate 2, not even sure what to say here, it's a superb adventure everytime I replay it. I've even got it installed atm, but my last run was like half a year ago and I got stuck (or rather, didn't feel like continuing) after proceeding to ToB and clearing watcher's keep.