Right, my best games have to be:
Doom II - Just for the double barrel shotgun
Half Life - For the sniping and camping ability with the zoomed in crossbow, and when you hit the wall beside someone and you got that "chink" noise and watched them crap themselves! hehe.
AOE II - Very playable, esp when your castle is defended by castles with 20 longbow men in each one!!
Duke Nukem 3D - Was just amazing for its time, with the strippers and the chain gun and the flying, ah, the memories!
Thats all folks.