If people are going to pimp Doom, then I'm sure going to pimp Rise of the Triad. It had the most utterly insanely awesome guns in...well, I don't know if any FPS has exceeded them yet. I hear that Serious Sam had some pretty crazy ones but I dunno. Guns that shoot four randomly wandering + heat-seeking missiles... guns that shoot one missile that splits into two when you let go of the button, which then seek out enemies... The most secret areas of any FPS I've seen, like, for serious. So many different end-of-level bonuses that they're on par with modern achievements. The Flamewall, which sweeps across the ground and is GREAT fun in Multiplayer. God mode, which is a powerup that makes you invulnerable and lets you annihilate things...for fifteen seconds. Dog mode, which lets you eat faces and make people explode by howling. Also, the first real video game age-rating scheme was released just before they went gold, so they had an emergency meeting to figure out how to make sure their game wound up in the highest classification. They added eyeballs to the gib effects.
It pre-dated Quake and maybe even Heretic. But it's totally worth it. Enormously long game, pretty challenging, surprisingly good and memorable music for being MIDI. And, well...if you remember what Apogee used to mean for games, that alone should be enough to try it out...it's funny without being dumb. Lightly campy. Hell they even used live-action actors for their enemies, which is totally more amusing when one of the bosses is a guy in a motorized and heavily armored chair. You just know that they took an armchair and taped paper to it. Man, what a good game. In terms of being the underappreciated but arguably better game, ROTT was to Doom what Unreal was to Quake.