1. Star Wars: Jedi Knight - Dark Forces II
The great campaign, the crazy gunfights, the campy live-action cutscenes, that AWESOME fuel-pipe level (level 9, still remember), the Kell dragons, the rail detonator gun, a god-damn LIGHTSABER, and so very much more. I played this thing way too damn much, both the single-player and multiplayer. One of the best games ever made for the PC.
2. Rayman 2: The Great Escape
Though absolutely infuriating in certain parts, this game still rests firmly in my memory as the best platformer ever made. The weird fantasy world, the great music, the varied level design, and the intense jumping kept me hooked for longer than I'd like to admit.
3. Icewind Dale II
I never actually beat this one (actually just started a new game a few days ago) but I had some great fun with it when it first came out. I made all my characters stupidly (I seem to recall having a monk that I multi-classed as a druid?) but that just made it even more satisfying when I managed to survive the brutal fights that ID2 throws at you. The "create your own party" element of this game was the biggest hook for me, and I'm kind of surprised that more RPGs nowadays don't give the player the freedom to create 6 unique characters for their party.
Oh, Gothic. I picked up my copy of Gothic as I was leaving Comp USA (remember that store?). I saw a single, lone copy on the shelf as I walked by, I snatched it up knowing NOTHING about it, and bought it. Little did I expect to then embark on the strangest, weirdest, clunkiest adventure I'd ever see in any RPG.
I'm not going to waste time describing Gothic, because you either know all about it already, or deserve to find out the way I did. It's a one-of-a-kind experience that has its ups and downs, but it will certainly stick with you for a long time to come.
5. Medal of Honor: Allied Assault
One of the first truly great FPSs that I can remember playing vividly. Who here can't remember the first time they played that Omaha beach scene? Sure, the enemies were cheap as hell (MP40s should NOT BE ABLE TO HIT YOU FROM 10000 feet away), the friendly AI was atrocious, and some of the levels were boring ad naseum, but the explosive action sequences made up for it.