JKK, I'd disagree with BJJ and street fights. It's great stuff (I've done it for a few years), but I hold the belief that 1) in a street fight, you don't want to go to the ground because more often than not, if you're on the ground, you have a couple guys on top of you and 2) it takes a long time to learn how to strike well in BJJ. I'll probably never pull a foot lock or an armbar rolling on the ground.
So, I'll disagree with the BJJ. I love it, but don't think it's the most effective. Most guys out there are unskilled fighters, so I'd take a basic self defense class. Unless you want to fight, and face the possibility of serious injury, my advice is to just walk away. Basic self defense classes give you confidence, just beginning to learn a MA and fighting in the street could get you your ass handed to you.
I personally think boxing has been the most effective thing I've learned in the few fights I've been in on the street, and BJJ is the best thing I've learned as a 'sport'. If you aren't used to really being hit, it's hard to gather yourself after being having a quick combo done on your face. And when the guy falls to the ground, it's time to walk away. I'm not going to seriously hurt someone for being a drunk or dumb.