The second greatest multiplayer FPS that exists, right below its predecessor.
The options may not be as robust as many shooters today but it is probably the most balanced game on the planet. The only thing that really matters is the skill behind the keyboard.
No other shooter has managed to capture and keep an audience like the counter-strike franchise, the whole thing has been going nearly 10 years now (I've played 1.6/CSS for 8.5 of em). The depth is beyond the design of many games today.
The graphics for it's time were rather good. People can custom build maps very easily and it has led to endless content. The sound was top-notch, there is no music but the sound effect noises are well done. The franchise is going strong after 10 years and will probably last quite a bit longer than that. This is the only game, of all games, that has so far been timeless.
To think it originally started as a user generated mod.