Your favorite other FPS games (old school single player)

So besides the obvious ones of Wolfenstein 3D and Spear of Destiny, which this thread is all about, what are some other of your favorite old school style (basically, just about shooting things) FPS games, or ones you have a fond liking of and commonly play? My list is of a decent length; ones I play only very occasionally are in parenthesis (and they mostly have common engines with bigger classics):

Ultimate Doom
Doom II (occasionally play Heretic as well)
Duke Nukem 3D (also very occasionally play: Shadow Warrior, Blood)
Quake
Quake II (but I don't play it as commonly as the first)
Descent
Descent II
(Serious Sam: First Encounter and Second Encounter 2001/02 Non-HD versions - I don't know why I don't play these games more as I love the style)

So that about wraps it for me, but how about you? I'd be interested to find out some of the common ones! Again old school only; nothing at or past Unreal/Half Life in overall complexity.

Except from Doom I and II, I didn't really play any old school FPS games.
By the time I was in elementary school, we already have RTCW.