Jiu Jitsu. It's not so good against multiple opponents, but if one opponent gets you to the ground you will feel comfortable there (That's where nearly ALL fights end up, whether you like it or not). Most likely, if you are female and attacked by a male he will be ttempting to get you to the ground anyway. Jiu Jitsu will teach you MANY ways of breaking (Yes, I mean bones) your opponent so you can escape. If you still can't escape, Jiu Jitsu will teach you MANY ways of choking your opponent so they are sleeping when you are escaping. The multiple attacker thing sometimes scares people off, but the other point to the game is to control your opponent. If you do that, you should be able to keep them between you and the other attackers. Another thing... If there are two, you can many time TOTALLY take out the first in under ten seconds using Jiu Jitsu, then it's basically two one on one fights. Anyway, check it out.
My roommate (girl) is in Jiu Jitsu. She's at practice all the GD time. She's about 5'7", 135. I'm 6'4" 190. I have so overwhelmingly little faith in her ability to get me to the ground, to break my bones, or to put me in some sort of Dragonfly Pincer move by which she can make me black out. That said, she sure thinks she can.
When we mock-wrestle, it's not a contest. She doesn't have the strength to match a would-be reasonable technique, and I don't know any girls (not to say there aren't any girls) who do. Other thing of note, and this is i'm sure her problem, is that she doesn't know how to throw a good punch, she's always trying to kick too high, and it would take two punches to flatten her (these are from watching practices) because she tries to block everything with exaggerated, sweeping arm movements. Again, I doubt that's universal in jiu jitsu, but she suffers from it.
IMHO with many girls in particular, they approach self-defense in a half-assed manner, looking for an aerobic workout. If you aren't going to commit to building strength and learning how to fight, there's no sense.
Browning 9mm is my choice, but I've been through the self-defense and handgun shpeel with various people so often that I'm going to shut my pie hole.