There are a few candidates..
Tempest 2000 is a no-brainer, it's pure fun... although i wish it would be harder..
I also love D2k, must be because I can actually play it (I suck at classic and plus though)
Battlemorph is great and also a HUGE game which you won't finish anytime soon. Although the water surface looks dull, the underwater graphics are actually quite nice.. and the music is great IMO
Alien versus Predator is good, although it's a pity it was stripped down so much from the beta (in the beta aliens could climp on walls etc) It also has an (artificially) slow framerate.. still it looks very sharp for it's time and the atmosphere is good..
I'd also like to name Rayman, it's just a classic, I enjoyed it so much when I was young (and it's still fun to play)