I'm going to be different and say UT2k4. :D

Where's the love for Duke 3D?
That game is, more or less, why I got an account here.

Same for me, but then I found there are many more cheap games... oh, yes, Painkiller also isn't bad, but feels more like Serious Sam with those hordes of monsters one has to plow through..

Rise of the Triad, closely followed by Blake Stone. : )

Old thread, but I'm gonna say Duke 3D. But with that said, I'm really looking forward to Blood tomorrow, and I've never played it. It may be better than Duke.
Painkiller was also really good.

UT99 is my favorite but Ive never played redneck rampage or shogo. Duke3d is also really good.

Duke Nukem 3D.

The UT games, then painkiller.

Duke Nukem 3D

I like UT4 my self.

I will thoroughly recommend UT2004, but mostly because of the vast quantity of mods available.
The single player campaign (basically a simulated multiplayer mode a ladder) is acceptable and the variety in maps is great, but add in Deathball, Alien Swarm and Metaball and you'll be entertained for weeks.

Blood, Descent 2, Kingpin, Redneck Rampage
That will probably change in the months to come.

Blood!

I will hunt you down, kill you, and then a year later I will raise you back to life, and you'll say "what was the fucking point of that?"

I can say "what was the fucking point of that post" right now. I don't have to wait a year.