BOII and maybe MW3 are your best bets for "older" games. I can confirm BOII still has a decent community, and although I don't know about MW3, I think it's something slightly behind that.
If things keep heading towards their current direction, although, I see Advanced Warfare taking the lead soon enough.
Except unoptimized drivers and memory leaks, the game's bugs and issues can be fairly overlooked without too much effort (they're noticeable, but not game breaking), and the game runs well enough on most systems, provided you "don't bite more than you can chew" in terms of game settings compared to your hardware. It just lacks dedicated servers, for which we already received a confirmation that they're going to come (albeit with an...