Half Life deserves my Hall-Of-Fame award for best First Person Shooter, ever. When it came out I was blown away. The facility that Freeman traverses through is so gargantuan, interesting and full of ominous, very creepy monsters. The story was great, the A.I. really great and the graphics and sound were unparalleled.
The single player campaign was so long and absorbing that I never have played it in multiplayer. Awesome game. Even now -5 years later- it's a better-than-average game compared to the others out there.