I have been involved with learning Aikido for some time. It is a decendant of Judo. I know from one of our white-belt classmates that it is useful. He was attacked by two guys who pepper-sprayed him and then grabbed him. When he was grabbed he threw the first attacker to the ground. Being thrown to the ground, when the ground is a concrete sidewalk, can be a serious thing. The other attacker came at him with the pepper spray again and the student grabbed his wrist, broke it, and put him on the ground as well.
The moves used in the above defense were simple, white-belt level moves. The key is that the martial art gives you SOMETHING to draw upon mentally, it gives more calmness and deliberateness of action. I believe ANY martial art will help in these key areas. The form of the art you should use depends on your size and strength. I chose Aikido because it was perfect for my wife's small size. Again, any art gives you a mental edge.
Just my .02