This question has been around forever and it will probably be around forever. The correct answer is always "it depends," followed by an interrogatory that seldom varies. Different people will find different answers based on traversing that decision tree.
Unfortunately, newcomers tend not to do two things.
First, they fail to look to see if the question has been asked/answered before.
Second, assuming they do find such threads, they seem not to believe them and feel the need to ask anew.
The question itself is so generic as to demand followup questions, as have been mentioned in this thread and by the OP.
1) What do you mean by 'self-defense'?
2) What is available in your area?
3) Are you willing to train?
As simple as these questions sound, answers still tend to vary.
I ask the first question because people live in different areas of the world, and face different kinds of threats. If one lives in a war-torn area with bombs going off and roaming armed bands of warlords, I suspect that 'self-defense' means something different than it means to a person living in a suburban zone of a modern first-world city that has a fairly low crime rate and 'self-defense' is more often going to involve random drunken barroom fisticuffs of the voluntary sort.
I ask the second question because there's no point in setting one's hopes on a style or type of training that simply isn't available where one lives. I am setting aside distance learning via book and video because that's garbage and doesn't work for initial training (sorry, facts hurt).
I ask the third question because it seems many fantasize about magic arts that give results without serious and continuing effort. The best way to learn to take a punch is...to get punched. A lot. The best way to learn to deliver a punch is to punch. A lot.
As to the art - meh. Horses for courses. I like mine, you like yours, and we can go around and around about that - there is no way to resolve that question to everyone's satisfaction.
But to cut to the chase; Isshinryu is best. You're welcome.