Good. No matter what I do I always play good. It makes me feel like too much of a jerk when I'm evil.
The one exception is sandbox games like inFamous, Red Dead Redemption, and Grand Theft Auto. Generally I'll always do my first playthrough as a good guy, (Though I believe it's impossible to do that in GTA...) and while I still feel like a jerk for it, I seem to be somewhat capable of playing the role of the bad guy in it.
Anything else... I just can't do it. No, I mean I can't. I tried to on Mass Effect. I made a character after I had done a couple of good ones, and I said that this character was going to be evil. Gave up before I even got to the Citadel because I couldn't do it.