Doom (id Software, 1993, PC). Predecessor Wolfenstein was cool (mein leben!) but doom was insane! Not just the enemies, but also adding verticality to the level design. Absolute classic from id Software.
Doom designer John Romero tells you all about the game while playing it - Most Blu-rays and DVDs these days come with filmmaker commentary tracks, but it isn't too often you get to hear a game developer give play-by-play while running through something
Doom, Quake creator John Romero making MMO first-person Shooter: John Romero discusses returning to the genre he helped create twenty years ago. http://www.digitaltrends.com/gaming/doom-quake-creator-john-romero-making-mmo-first-person-shooter/