Are we counting mods and expansion here?
If so it's Half Life by miles and miles and miles...
Counter Strike, DoD, Hidden, Gary's Mod, Minerva, Neo tokyo, Insurgency, Perfect Dark, Goldeneye, Synergy, Zombie Panic!, Stargate, Plan of Attack, the Ship.
The number and breadth of truly excellent pieces of content that have sprung out of Half Life is unrivalled. Half Life itself isn't too shabby either.
Against that Quake and Battlefield games come close, especially Quake and it's seemingly infinite modsphere.
Never mention Call of Duty as the greatest series ever, that's like saying Michael Bay should have an Oscar or two.