Go CS Source.
I use to play the CS Condition Zero multiplayer which i think was an updated version of 1.6 - Condition Zero had a single player Campaign as well which was pretty suckful - your finish it in 5 - 6 hrs, and it had a single player skirmish missions with the bots which was better than the campaign and warmed you up before you go online. But the multiplayer code is good and still is used by many today - i wish i never sold the game as i still have an urge to play a fast action shooter thats blardy addictive as HELL!

. But if ya want the better graphics and latest updates go with CS2 SOURCE.