Ok I've played all 4 of those games at least a couple times, some a lot more (MW and MW2).
CS:S- Its a good game which will be populated for a very long time but the community is douchebag and nerd central. Also, since the game is so old and has been actively played for so long now, there are too many skilled clans for us to start out fresh and actually enjoy/dominate the game.
Cod 4 MW-
Very fun and for the most part balanced game. Nade and noob tube spam ruins it somewhat but that can be fixed with promods. Unfortunately regardless of what xfire stats says, the game is grinding to a slow death and the last thing we need is to move to another dying/dead game.
Cod 6 MW2-
Been playing this a shit load. Ok because of IWNET and the lack of dedicated servers, this is really a love/hate thing. I mean if you get a good host, no hackers, none of the bullshit, annoying, overpowerd classes (danger close/OMA noob tubers, Commando knifers, akimbo g18 spam kids) and teams that play for objectives, it is a VERY VERY VERY fun game. I mean it is fun as hell. Unfortunately, this rarely ever happens due to the IWNET bullshit. Also Playing in a party with other clan members makes this a LOT worse because it takes forever just to find any game at all, much less a game meeting the previously mentioned criteria. The game will never amount to anything competitively as long as Activision refuses to put in dedicated servers, which they wont. Another thing is that there will be DLC coming soon and all sources point it to be being paid (15 dollars but not confirmed). The game costs 60 dollars which is restrictive enough but 75 dollars for this REALLY???? Also, weapon balance is out of wack. Assault rifles are overpowered, sub machine guns and LMGS are good and sniper rifles are underpowered.
BC2-
I preordered this and tried it out before canceling my order. It didn't seem anywhere as much fun as the gameplay videos made it seem but I suspect this was because I was playing with randoms. I hear teamwork is really important in this game. Also the weapons sounded like garbage and couldn't cause damage worth shit but I hear you get better weapons later. Another thing is that the game was poorly optimized for performance in the demo. I don't know if its been fixed now, but it needs to be.
Anyway tl;dr, I vote for BC2 because it is the newest and least annoying of the 4 games though I enjoyed it the least. I will buy it if the clan comes to the decision but I wouldn't mind if any of the other 3 were chosen either.