Got a lot of games but here are a couple of favs:
Quake 1 (you can tell by my avatar
)
Unreal 1 (NOT UT, the Single Player Unreal - very atmospheric)
Unreal Tournament (yes UT as well :p)
Duke Nukem 3D
Descent 1 & 2 (wish there were more 6DOF games. There was Forsaken, but that was meh)
Half-Life
Heretic
Hexen II
Hexen
Diablo 1 (Excellent atmosphere and music, can't say D3 will have me nearly as emotionally invested, because it's missing the composer from Diablo 1)
The Elder Scrolls II: Daggerfall (I played this after oblivion and morrowind, when they released it for free and it absolutely blew my mind)
Prince of Persia (the Original, not sands of time. I like having no powers better)
MGS1
Battlefield 2 (favorite MP game with friends, it was amazing)
Crysis
Jedi Knight 1: Dark Forces 2
Battlefield 2142