There really isn't one specific map I always vote to skip. For me, it comes down to my mood that day, and the kind of player I want to be at that given time. For instance, if I want to snipe, I'm naturally going to vote to skip smaller maps, as that will make sniping harder for me to do successfully. Or, if I want to get in close and do some knifing, I'm naturally going to vote to skip a map that is extremely large, and a good place for snipers.
If I had to pick one map that I really just don't like though, it would probably be Overgrown. I wasn't a fan of it in Modern Warfare and I'm not a fan of it in Modern Warfare 2.