The original Doom is the greatest game of all time
The basic concept of this game is to find keys to open doors, which leads you to the exit of each level. The 'find key; open door; repeat' mechanic may seem simplistic, but Doom is actually a fairly complex and difficult game. The puzzle elements can be quite difficult at times, and the countless bad guys that try and ruin your day can make it even tougher. There is a sense of 'depth' in every single level in Doom that seems to be lacking from many modern FPS games - in all honesty, I can't think of any other shooter game I've enjoyed more than this one.
Originally, Doom consisted of 3 episodes (9 levels each), but this XBLA port also includes a fourth episode. Let me say from the get-go that although the first 3 episodes can present a challenge on the default, 'hurt me plenty' difficulty, this new fourth episode is an absolute NIGHTMARE on the default difficulty! And when I say that it's a nightmare, I mean it in the best way possible - this episode is a hell of a lot of fun, and makes the game entirely worth re-purchasing for fans of the original.
Doom is a game that I have dumped countless hours into (on PC and XBLA), and I intend to play it even more in the future. This is a revolutionary and virtually flawless game, and still stands today as my all-time favorite video game. People who only appreciate the newer Call of Duty games may have a tough time playing games from the stone age, but any open-minded gamer needs to check out this masterpiece. It doesn't get any better than this, and the Xbox Live port is a faithful recreation of the original mayhem. The controls are smooth, the puzzles are extremely satisfying, and the level design is the best that you'll ever find in a video game.