Quake 2 is simply a classic, and one of few games I've personally mastered. Quite literally, the moment achievements come out on the game, if they ever do, I'll be among the first to get them all.
Quake 2 is basically a game where you infiltrate and alien world in order to kil their leader and make way for a larger-scale attack, while deactivating defense grids and blowing up a giant gun battery. You also kill things.
The game has very little story, delivered in the manual of the game (which i don't think comes with the game), but basically you're a soldier who deploys in a drop pod to the world named Stroggos. Your pod gets hit hugely off-course, and you land in an outpost of not much consequence.
You are totally alone. No buddies, no air-support, all you have is your blaster, and your PDA which give you stat readouts on the area (like how many people are still waiting to die, or how many secrets) but more importantly, your objectives beamed down from the capital ship in orbit into said PDA.
The game is essentially as good as FPS adventure action games get witohut going into arena FPS territory, and is (in my opinion) the only one to do this to date.
The game has level stats exactly like in Duke Nukem 3D, Quake 1 and even Doom 1, and the secrets are always really good and memorizing their locations can become second-nature and are even instrumental on Hard+ difficulty.
Quake 2 is an FPS that can be played mainly as a singleplayer game, but has Multiplayer and even Co-op, and lived in the days when Balancing wasn't even a matter that was at all important in the eyes of Developers.
Multiplayer is essentialy arena-FPS and that's awesome.
The game has 3 mission packs, 2 of which are on Steam (and i think the 3rd is too).
For anyone who likes FPS games that doesn't easy-mode the experience by giving you regeneration, checkpoints, and newly refreshed ammo and weapons at the beginning of every new level, Quake 2 is a must have.