Counter strike is the daddy of all fps games
Most players, most servers (I find like 50,000 when I search, compared to like 500 compared to half life 2 deathmatch, or like 200 to left 4 dead)
And css isnt about graphics, i mean, I would play COD4 online all the time, its a bit like counter strike, but much better graphics, but I still play css a lot more than it, plus in css you have all these mods, zombie mod, gungame, deathmatch, slide, etc which is just endless fun