This is one of the earliest games I remember playing as a kid, and BOY did it leave an impression. It's obviously not the first FPS game, but it is probably the most successful of the earliest in the genre and remains one of the very best. It is a smooth, simple game that has some of the best level design in gaming. All of the weapons are cool, the enemy design is incredible, and the FPS genre owes so much to everything this game pioneered. Special kudos to the music and sound design--that door opening/closing noise lives in my head rent free.