Here's the games that I have the best memories of, and not one's that I play over-and-over, because they don't seem to stand the test of time:
Diablo : I still remember seeing the first advert for this one in Mac Addict magazine. I waited 9 months for the damn thing to be released and when it was I spent that Spring sitting at my PC. I still associate Easter with Diablo, I went through the leftover Easter candy trolling the depths!
Baldur's Gate/Icewind Dale/Planescape Torment : All share a concept of immersive story-driven RPG. Loved them all!
Myst : Worked on this with my roommates at the time, and I still think about being stuck at one point for days, with none of us able to get beyond it. At work, it occurred to me that things were awful dark where we were looking. Called my roomie and told him to turn up the gamma on the monitor, and Behold! There was the lever we were looking for, hidden in the shadows where we couldn't see it!
Curse of Monkey Island : Another all-nighter with friends. Guybrush Threepwood was too damn much fun. Who would have thought I'd have to give that pirate a wedgie?
IL2 Sturmovic : include all iterations here. Spent years flying WW2 fighters with a great squad of guys from all over the world. Met some lifelong friends, and for a while could tell you engine specification differences between the FW-190 A6 and FW-190 A8, muzzle velocity of the MK 108, and why the BF 109 was the workhorse but the P-51 changed the war. I swear I could take off and land a Cessna after everything I learned in that game!