Doom (or DOOM) is legit in the sense that it was one of the first FPSs (if not THE first) that I ever player. Although I've never had much love for the genre, I love this one for nostalgia's sake. The graphics weren't anything to really speak of, and the plot was meagre at best, but what Doom had that other games of its time didn't have, really, was the ability to make me feel fear. Not only is the landscape eerie (and not just because it is overwhelmingly over-pixelated as all games of the time were), but the creatures were frightening—from zombies to the demons of Hell, Doom had it all.
There were, if I recall, 3 levels with 9 stages each, playing on the holiness of the number three (and multiples thereof) in the Catholic religion, which was fitting considering the last level was Hell. The way through each stage was through a hidden door or by finding the bright Exit sign after killing all of the monster. Again, I'm in it for the nostalgia folks. There's not much that can be said when a game looks like this:
Despite the game not looking the best now (even though it did then), and having a minimal plot, it still paved the way for many of the great games today. And, for what it lacks now in my mind's eye, it made up for in awesomeness then. As much as I can hate on it now, in 1993 when it came out it was THE TITS. Let's just take a second to recognize all of the awesome things that came along with Doom:
-The ability to rack up SO MANY WEAPONS. Can you imagine a game now where you couldn't switch between you rocket launcher, rifle, pistol, cannon, shark-shooting-gun/whatever? No. You can't. Thanks for that Doom.
-Shareware games. WHAT AN AWESOME IDEA. Nobody does anymore (or ever really did this...) because everyone just wants to make the most money. Luckily, Doom released this game in pieces, and encouraged other gamers to share. This, I think, laid the foundation that so many of us enjoy today in our community. Just in case you wonder what it looked like, you can get the original Doom here.
-John Romero, the designer of the game played it with IGN recently. Which is both humble, rad, and awesome to watch in my opinion.
-You were able to customize the game if you were interested in doing so because of so called "WAD"s. Basically, players were able to mod this game however they saw fit, and created awesome, new versions including a Simpsons, Ghostbusters (below) and, later, a Pokemon modded game.