I agree with M_Cep - you simply cannot say "this is the best martial art" because there are too many factors involved with the decision. Example - a physically fit male in his 20's can perform some of the arts mentioned here such as Krav-Maga or Ninjitsu. However, that won't work for a woman in her 50's (nor for the guy in his 20's when he's 80!)
For long-term self-defense, it would be much better learning an art that stresses using the opponent's physical strength to their disadvantage such as Jiu-Jitsu, Aikido or Hapkido - what are known as "soft" martial arts. They are more geared for women and the elderly, but are equally effective when used by anyone.
The others mentioned above as well as Karate, Tae Kwon Do, Tang Soo Do, etc. are "hard" styles that are more suited to those physically fit, or who desire to be.
However, nothing you learn matters if you don't know how to develop the proper mindset for combat - all the techniques you know won't do a bit of good if you're frozen in fear or if your adrenaline is rushing so bad you can't perform a move effectively.
Of course, nothing beats a gun, but I wouldn't consider that a martial art!
Hope this helps you find your way...