DUKE NUKEM 3D / 3D Realms / PC
Ahh, the 90s. With just some moxie and a bunch of one-liners ripped off from Army of Darkness, you could take on the world.
Duke 3D was the big third step in the progression of the FPS genre (Wolfenstein 3D and DOOM being the first and second.) It wasn't terribly different from DOOM, but did everything bigger and better - more expansive and organic levels that didn't confine you to one vertical "layer" at a time, the ability to fly with a jetpack, the ability to swim and go underwater. Hell, being able to jump and use PgUp-PgDown to look up and down 90 degrees was considered a huge, revolutionary deal at the time.
Duke 3D is also a time capsule showcasing the best and worst of 90s Western game design mentality. It continues in the footsteps what I've fondly referred to here before as the Romero School of Level Design - an emphasis on cheap deaths that promotes a constant F5/F7 quicksave/quickload mentality. Enemies illogically spawning in, hiding in darkness, combing labrynthine levels for keycards, random button-press puzzles with no apparent logic or reason. But it also shows off one of the best things about the 90s - the anarchic, "just do stuff because it's neat" approach of what were essentially a bunch of frontiersmen carving out a new genre and a new market. Not just the un-politically-correct attitude merrily stomping around giving no fucks whatsoever, but the stuff thrown in purely for shits and giggles that tell you that a handful of idiosyncratic human beings with personalities made the game instead of some Corporate Marketing Department.
Most gamers' fond memories of the game are probably predicated largely on modem-or-LAN Dukematches in the first 3 or 4 levels of the first episode. For the single-player, the initial campaign (distributed freely as shareware back in the day, when PC game demos could be pretty goddamn substantial) gets off to a very good (if darkness and spawn-in-heavy) start. It then kind of falls apart with the requisite terrible water level (at least there's an excuse here, as it was pretty much the first in an FPS) and the canyon level filled with obnoxious platforming jumps and generally suffering from unclear direction. Unfortunately the follow-up campaigns tend to follow the same pattern; a couple of great-looking and engaging initial levels followed by a lot of overdifficult tedium to slog to the end.
Control is also a bit of an issue, at least through the 2013 lens. You can reconfigure the crappy DOOM-default controls to a somewhat more comfy WASD style, but gamers used to BLOPSing it up on a controller and can't handle anything else are just going to be shit out of luck here.
I've noticed a trend recently (in certain quarters) of romanticizing 90s FPS, in reaction to today's overly scripted and cover-based AAA shooters. I think that largely comes from 10+ year old memories of the good times had with these games; play them now and they're pretty hard to take, with their own share of significant flaws. To a great degree, Duke traded on simply being the first to do a whole bunch of things; all things that have been refined and done much much better since. It was mindblowing in 1996, and those of us that were there will always have the fond memories of that time ... but I dunno how much value it has left beyond that.