Half life 1 and 2: I must have replayed these games like 10 times each.
The left 4 dead franchise: its unbelievable how addicting this game is, even with random people. Memories of long, 5 hour expert realism attempts and hilarious versus antics will probably remain with me for a long while.
Morrowind: Even after completing nearly everything vanilla had to offer back in 2004, I still return to the game to this day, enjoying both the mods and the original content which still stays enjoyable.
Gauntlet Dark Legacy: old game released back in 2002 for the last generation of consoles (originally an arcade game). Six years ago I actually lost my gamecube disk for the thing, and the game was already so forgotten that no game cube copies remained. So I just went out and bought another copy for original xbox (the controllers were better).