The best shooter from the 90's.
With this in mind Half-Life is without a doubt a game that re-invented the FPS genre. The storyline is excellent and intriguing for it's time. It almost feels real. The use of puzzles perfectly fits with the mission goals of the game. Not only that the levels are quite varied and interesting, no longer are you stuck in small corridors. It's a mix of outdoors, indoors, and an alien environment.
With respect to the graphics, it's no slouch from 1998. While it used the Quake 2 engine in the original game but it also introduceed a new thing called "skeletal animation" that was something innovative for it's time. The graphics and textures and the various environment makes this game look quite good for it's time.
This is a game that no First Person Shooter fan should miss.