Best there is

User Rating: 9.6 | Counter-Strike: Source PC
If you're looking for an online shooter, CSS is basically the only choice. Everything is top notch about this game. The source engine makes the graphics truly something to see (if your computer's a beast) and all the guns have their own unique sounds. It's possible to tell what the enemy has from across the level. If you want to play a game competitively you might want to look to 1.6. CAL for source is rubbish and the only hope of saving source is CEVO. But, if you don't care about competing in a game, CS:Source is just a truly fun online shooter.