What are the best ways to train for self defense?

What martial art or fighting system would you recommend, and are there any online resources for learning them?

What muscles are most important for self defense? Would it be best to focus on a certain area or to do a more whole-body focused routine? Thanks in advance.
Update: What martial art or fighting system would you recommend, and are there any online resources for learning them?

What type of exercises should I focus on?
Update 2:
5 answers 5