Simply the best FPS
My first experience with FPS was with Wolfenstein 3D. I liked it alot, but I couldn't play for than 20-30 minutes at a time without becoming nauseous and/or getting a headache due to the game engine mechanics. This remained true for all ID games. I never experienced this with the Unreal engine. The first time I played Unreal I was hooked. A buddy of mine and I would get together for multi-hour sesssions progressing from one level to the next. The open environment, allowing the player to wander Na Pali nearly without limit was ahead of it's time.
Unlike the author, I thoroughly enjoyed the music. Sound effects were also outstanding. There was one level where you had to go down a corrider to retrieve something from a room at the end. When you returned, the lights started going out sequentially before you, darkening your return passage. This was where the flares and flashlights you collected previously came in handy. Still, the moment the Skaarj emerges from behind a hidden doorway and engages you in close combat just as you are nearing the exit and you think you're home free, is utterly unforgettable.
The shear number of levels in the game, I think there were 36 in all, was unmatched by any other game I ever played. It took me months to complete the game, though I actually never made it past the last level (in the Skaarj mothership). I kept getting trapped in a small room with hordes of Skaarj coming at me everytime I opened the door. Even using the unlimited ammo cheat I could never kill enough of them to escape. My buddy claims to have finished the game, but that was sometime after I'd given up and moved on to such titles as Mech Warrior II and Alice.
The entire Unreal series is simply amazing, though I never played Unreal 3, as it was never released for Linux. I still play Unreal Tournament 2004 regularly. The AI was unmatched in it's day, and I expect is still better than many even today. I only play single-player mode because most of the time network latency makes the game mostly unplayable, and single player can be modified in almost every respect to change actual gameplay. Most playable game ever.