I know, I know. Everybody likes this game.
But there is a really good reason... It's fun.
I read a review on another site which criticized the story of "Half-Life." The idea the reviewer had was that, although it is a good action/platform game, it doesn't deserve the avalanche of praise it gets for it's story.
I think this is an interesting point, but it is not entirely true.
Obviously, there is not much of a story in "Half-Life" when compared to adventure games with deep stories complimented with a plethora of text or even when compared to more recent first person shooters like "Hale-Life 2."
On the other hand, "Half-Life" does have a story, it's a fun story, and it's extremely well implemented in the game.
This cannot be said of other FPSs of the day. "DOOM" doesn't really have a story, and, "Quake" doesn't really have a story either.
Furthermore, "Half Life" tells it's story through the environments of the game. You live through the story, rather than read about the story.
I believe that Valve could have thrown in some cut-scenes and possibly wrote some discoverable text to supplement the story if they wanted to, but it would have broken up the pace of the game too much.
The story of "Half Life" is it's back-story, and the story of the events that you unfold during the game.
Obviously, this is an early attempt at this approach, and it's not perfect. Also, the technology was not refined enough to have the kind of story-driven NPC interaction that is present in the sequel, but it's a really good start.
So, I have to mention "Half Life 2" here, of course. "Half Life 2" is one of those games that is larger than life. Some folks have complained about the dorkiness of the main character, or expressed a general disinterest in the Science Fiction theme of the game, but those people were never going to like this game, no matter how good it was. To everyone else, "Half Life 2" is the one of the best FPSs ever made. "HL2" picks up where the first game left off and ups the anti with vastly improved graphics, and a brilliant physics engine.
My only complaint with HL2 is the anti-gravity gun, it's just too easy to kill people with. About the only time it isn't useful is against those giant spider things and when you have to fend off large numbers of combine troops at the same time. Regardless, this is really fun game, with less bugs than most games and incredible replay value. Both "HL" games get honorable mentions for "extremely high fun factor."