Doom. You can shoot a variety enemies from the first person perspective with a variety of cool guns. The best thing I had ever seen easily.
Quake. OMG, like Doom but even cooler. You can even go underwater. And NIN music, are you kidding me.
Duke Nukem 3D. Like Doom and Quake but with an inappropriate protagonist with funny sayings.
Unreal Tournaments (various). Takes everything to an even higher amped-up and cool-looking level.
Half Life. The best video game of all time, now and forever.