Probably Call of Duty.
Good community, great games, etc...
Half-life Halo Call of Duty
Halo Battlefield Call of Duty
No Unreal Tournament? Medal of Honor? Not even an all-encompassing "Other"? =p
My vote would go to the Medal of Honor series for campaign purposes, although multiplayer is quite fun as well. Unreal Tournament wins it for multiplayer, it's the classic arena shooter.
And GTA games are not first-person shooters. Hell, the first two were top-down, and the modern ones are third-person.
^This man has a point
Also . I voted Halo...
Also . where is Mass Effect?
The Half-Life series is one of the greatest series in gaming pretty much ever, so it get's my vote. Even humble ol' HL2:DM is fantastic.
Though yeah, there are several series missing from the list
Bioshock man. Its a series now that it has BS2.