Even as a CS:S main, I think TF2 fare better, followed by CoD or BC2.
As of now, the TF2 division has the greatest number of active members. So if a robot zombie apocalypse begins, there is some strength in numbers. And with the variety of classes you have almost everything covered.
Only con for them is that people would be too busy trying to pull off weird or crazy shots (or taunt kills) and it may get overrun quite rapidly.
CoD has claymores/bouncing betties, nuff said. And with MW2 perks (theoretically, just lumping the 3 together), just get OMA and spam clays...
BC2 has vehicles. Nothing does a body good like being safe in some armor. Or a chopper. Or a building that will fall apart when someone coughs. Wait, scratch that last one.
As for CSS, a well combined defense will be impossible to penetrate. Just think of bombsite B in Dust2 or the projector room in Office. And if we have AWP... well then CSS goes to number 1 cuz it's gg. But seeing as how AWP is disabled, we are #4.