GilesHabibula: 4. Jedi Knight: Dark Forces 2 (these two games had possibly
the best sense-of-place in a gameworld ever. Also, DF2 gave me awesome vertigo many times.)
^^ This, a 100 times!
GilesHabibula: Yes, I included Shadow Warrior and Blood, but did not include Duke Nukem 3D. While I loved Duke Nukem, and spent more time with its level editor than any other game, I just didn't care for its level design.
Duke 3D didn't fully open itself up for me until I tried multiplayer. At first sight, one would think that these levels are more geared towards singleplayer but the level design is perfect for multiplayer.
Levels like Tiberius Station and L.A. Rumble are absolute genius and I didn't realize this until I spent countless hours of null modem / LAN sessions playing Duke against 1 or more friends.
Steroids + jetpack makes for some of the fastest and craziest FPS gameplay of all times to this very day, insane really.