It's gotta be DOOM! The first game to introduce me to PC Gaming outside of Minesweeper and Solitaire!
Gaming
Probably Lotus Racing (ran off one floppy), then Quake 1 and Carmageddon 1. Never really gamed much after those few.
I got into ZZT, a "topdown" 2d game with its own map editor and even a rudimentary scripting language. Stumbled on it because I wanted to make games. People made some seriously impressive shit on that thing and its successors.
Heroes of Might and Magic II, I still will fire it up to play occasionally! I have so many fond memories of me and my dad playing into the long hours of the night.
Xargon and Highway Hunter.
Ultima Underworld. Bought my first sound card for this game.
Theme Park. So classic.
Space Hulk would definitely be mine
Going by most influential, it'd have to either be Doom or Ultima Underworld. Both of these inspired entire genre's that still live on to this day (FPS and First-Person RPG's like The Elder Scrolls). Personally? Ultima 7.
Duke Nukem II. Also love the first one. Apogee in general made some really excellent DOS games.
Star Control II for sure. Fantastic game, still one of the best story-based games I've played. Heavily inspired a lot of more modern franchises, too, including Mass Effect.
Doom, Duke Nukem 3d, Zak McKraken and the alien mindbenders, One Must Fall 2097
Crusader: No Remorse/No Regret
Sometimes I wish they made an FPS reboot, then I remember EA owns it.
Gunship 2000. Shit was amazing.
Ultima series.
Every once on a while i get back to Mad TV. Casual game before it was cool. Also Indiana Jones: Fate of Atlantis
Quake.
Quad Damage intensifies
> bind q "impulse 255"
Supaplex and Gorillas are my go tos!
Hard to say. Secret Agent, Treasure Mountain, Azrael's Tear, Chill Manor, etc. All solid games. Hard to pick just one.
Sid Meier's covert action. Absolutely amazing spy game with lots of minigames and a randomly generated hierarchy of bad guys to catch every mission. Really want a remaster but I don't think I'm getting one.
Star Command was my favorite. Loved recruiting crew members, buying and upgrading ships, and going around the galaxy to complete missions. It's the game that got me hooked on RPGs as a kid
Probably X-Com: UFO Defense. Got really into it and holy hell got angry at that game. But damn it made me want to play more!
I never had a DOS computer back then and only played a few games through emulators in recent years. It has to be DOOM for me.