HL1, and it's variants, like HL: Blue shift, where you play as the security guard that helps Freeman, and HL: Opposing Force, where you play as the guys going after Freeman. That, and of course CS 1.6!
ive been on a Homeworld binge lately. The original Homeworld was released in 1999 by relic, same company that did Warhammer 40K, and is doing Company of Heroes. Amazing game. has anybody here played homeworld?
CS 1.6, all the way! And Unreal Tournament (The original). They might be getting on a bit but for me they just feel like games should. Pretty rough graphics wise but it's all about the gameplay. No advanced physics engines or bullet time effects, just shoot what you see and watch it bleed/explode unrealistically.
That and Rollercoaster Tycoon though building theme parks is only fun for so long.