If I had to choose one for self defense it would be... a hybrid karate system like what mord said like ACK, Jeet kune do, Krav maga,Kenpo, Kajukenbo, or a traditional Japanese jujitsu system.
I do not believe one system is superior to another, you get out of it what you put into it, and keep an open mind. I believe in building a strong foundation in something and then adding techniques from other systems, that way you are more able to adapt to different types of people and take them out of their element. IMO.