In my opinion, the absolute best FPS of it's type. While it may not have the complex, involved storyline that Half Life made the norm; the atmosphere is one of the most well-defined and fleshed out in video gaming history to say nothing of the fact that the gameplay is some of the most enjoyable.
In the days before computers could handle chucking waves after wave of enemies at the player; level designers had to much more smart with how the number of enemies they could have were placed; and this game takes that philosophy to it's most deliriously satisfying incarnation ever seen: zombies burst unexpectedly out of walls, gargoyles suddenly spring to life and mad shotgun-wielding cultists with disfigured faces fire madly at you from a train station ticket booths while screaming obscenities at you in mock-latin. It's this mixture of insanity and internal logic that makes this game such a success; everything you encounter from the barrels of human heads to the terrified victims of the cultist's rituals that run around blindly until you kindly put them out of their misery by setting them on fire makes some kind of twisted sense in this over-the-top, ghost-train universe the game takes place in; and the exceptional, glorious, beautiful level design services that universe and the refreshingly diverse locations within it, perfectly.
On top of all this; the game is lovingly loaded with extra little details: Caleb's gleefully sadistic "T-T-That's all folks!" after he blows up his enemies with dynamite, the ingeniously placed secret locations, decapitating a Zombie with a pitchfork and kicking it's head around the level never seems to stop being fun... Really, the only downside I can imagine from playing this game is that the environment's are so rich and interactive, the setting so memorable and the gameplay so lovingly crafted that the likes of Halo and Left 4 Dead may well seem begin to seem lacklustre by comparison.