Single player for me.
Honestly I have been kinda struggling with the whole multiplayer thing. It may just be bad luck, but I continually have problems with the people I end up playing with.
Example 1: I was playing Halo 3. On one of the maps I grabbed the rocket launcher, and fragged a couple of people and than moved on. At the end of the game, one of the players started to yell at me, claiming that I was cheating, and that using the rocket launcher was wrong. Now keep in mind there where never any discussions before the match, just a standard multiplayer deathmatch.
Example 2: I was playing MW2. After recently friending one of my teammates, he decided to test my skills in a one on one match. We played for a while, and there was no contest; he is much better than I am. Problem is every time I got a kill in, he began to yell at me, saying that I was camping, using noob weapons, etc.. and this was in a one on one match.
Example 2: I was playing the Reach: beta. Once a game was complete, I received a series of messages from one of the players. Apparently I kicked him, with out knowing it. This persons response was to swear, threaten, and ultimately . When trying to apologize for what I may have done, I only got the response that I was lying and some more expletives. It's a beta. Shit happens. I don't think that anyone can control who is kicked or not. The worst part is that any progress that you make in the beta won't transfer over to the actual game, so any stats you acquire don't even mater.
I know that I am playing popular games that bring a large crowd of people. I just seem to always end up playing with doushbags, or adolescents that are more interested in keeping their k/d ration up, than having fun. Co-op is usually fun, but the same issues of "I want to win, vs lets work as a team" always come up. The above examples are only a few in the many that have happened to me, but the final result is I look forward to, and spend more time in the single player aspect of any game.