Quake
Half-Life
Unreal Tournament
to this day i still return to quake.that game has got something special

back in the day it would be duke nukem 3d or quake 2, now after all these years i still play doom1 and 2 the most.

Dark Forces

Aliens versus Predator 2
Dark Forces 2
Doom

Gameplay: Doom II
Gameplay, Honorable Mention: Quake
Level Design: Dark Forces
Level Design, Honorable Mention: Memento Mori 1+2 usermade .WADs for Doom II

One thing I loved about the DOOM games was that you could get the enemies to fight each other. So despite DOOM being a "mindless action game" it is actually possible to play it "cleverly".

Aww man! I can't believe I completely forgot about Dark Forces/Jedi Knight games. I own the entire series in original big box, with the exception of Dark Forces 1.

Wolfenstein 3D was my first FPS, and it certainly holds a spot dear to my heart, but there's nothing like the replayability, story, gameplay, multiplay, single play, modding, etc., etc., etc. that Rogue Spear and Raven Shield offers. I know you can't really compare these two to games of Wolfenstein's day, though.

Anyone directly familiar with the SWAT series?

Funny. Deus Ex is a title I'd not consider a "real" FPS because of mainly being an RPG. :P

Personally when I hear "oldschool FPS" I think of Doom era and Build Engine titles, normally I think of Half-Life as the turning point when FPS games became "modern". Some titles released after HL that stick to simpler conventions do make my "oldschool list" though.

Aaanyway (in no particular order):
Doom II
Duke Nukem 3D
Shadow Warrior
Blood
Outlaws

More retro than oldschool (IMO):
Painkiller
KISS Psycho Circus: The Nightmare Child
Serious Sam: The First Encounter

Doom II, just for the amount of hours I sunk into. I even found some chintzy, level-creation software at a yard sale and could use it to generate new levels to play. Just hours, and hours and hours of playing that game.

Also, Duke Nukem 3D and Hexen were up there... but nothing touched Doom II.

You prefer it over The Second Encounter?

I do, personally (I like the clean simplicity of everything), but I thought I was the only one

I thought old-school FPS were the ones with sprite based objects and 2.5D engines.

Yeah I do. Maybe it's because on my initial playthrough of Second Encounter I was bored because I felt like I had just recently played the same game. But I also think that the additions they made hurt the solid design of the first one, much like the new weapons of Battle out of Hell didn't fit in with the perfectly designed simple set of guns of the original Painkiller.

Fun fact: I had beaten The First Encounter with a friend in splitscreen coop mode and when another friend with a sniper fetish asked us whether there was a sniper rifle in the game we both were like "A sniper rifle in Serious Sam? Please...". We couldn't trust our eyes when we played The Second Encounter.

I agree on both counts.

In fact, I often end up feeling like that about expansions and "the same game but with more stuff" sequels