depends on what your body-type is..
if your short, jujutsu is the way to go because the ground can be an equalizer in terms of going up someone bigger, and because of leverage.
if your taller, usually striking arts are better, the farther away your opponent is, the better..
but no matter if your short/tall, you want something well, rounded and reality-based... something that teaches you striking skills and grappling..as well as dealing with real-world scenerios.. fighting with/against hand-held weapons, ect...
usually karate is not an effective self-defense system, as it has been watered down way to much, is not practical for street survival
krav maga ws stated above and I second that, its what the Isralie(sp?)
army teaches to its special forces... based on real-world applications and techniques... how to use everyday objects as weapons...pencils, keys, umbrellas...ect.. the only hard part is finding a good qualified instructor
I would also recommend Muay Thai as a self-defense art, while bred for the ring now, it is still a very dangerous system that teaches how to inflict massige damage on an opponent in a very small amount of time... most places that teach this also teach jujutsu with it...
just run a search on the net for whats around you... and be sure to include the words "real world self defense' in your search or you'll get all these tae kwon do and karate dojos that only teach kids and crap that will get you killed in a real confrontation.
Best,
Jin