As Troy said though, the best gun for self defense is the one you will have with you.
So, make sure the one you have with you is one which will do the job.
Too many shooters confuse self defense with killing someone. That's not the primary purpose for self defense. The idea is to stop your opponent's aggression, which doesn't always translate into a death. Sure, a mouse gun is better than nothing, but it takes several minutes for an adult to bleed out and die. In the mean time, someone with illegal drugs on board, or who is emotionally disturbed can do a lot of damage to you or your loved ones before they eventually bleed out. There are many options with modern firearms that allow us to carry a major caliber in a small, powerful package. Expensive? Yes, but how much is your life worth?
Then there is the selection of self defense ammunition...........
(http://i204.photobucket.com/albums/bb294/pioneer461/police.png)