I have seen this question pop up so many times on martial art sites. Many people have their opinion and others just laugh at the person who posed the question. The answer is simple. The best martial art is the one you are training in. If you are not training and looking to begin, then the best one is the one you are drawn to in the movies.
Here is the thing about martial art schools. They are not all the same. The instructors make or break the school. So, if you are looking for a school to start training in, then I would recommend trying out a few different ones around town. Go to each one for one to four weeks and see how you interact with the teacher. Many are great practitioners of the art, but not good teachers. Others are good teachers, but not very good at demonstrating the techniques.
It is important that the teacher and you get along. What I mean is that the way they teach and the way you learn make for a good match. This will make all the difference in how much you learn and at what speed you pick it up. Going to class is not always the most fun thing to do, but having a good teacher makes it easier. The classes should be hard and full of physical challenges. At the same time, they should be interesting and have something that draws you in.
Once you find a good teacher, then study every chance you get. Become extremely proficient in the art you choose. Having basic moves mastered and more advanced ones practiced is important to understanding your art and your body. This allows you to make a good decision down the road, as to what you want to do when it comes to staying with it or moving on to another discipline.
Many students start off with karate or taekwondo and find that they absolutely love the art and want to continue. Others find that they have learned basic techniques and realize that their bodies prefer different movements or fighting styles. Your body will teach you a lot about what it can and what it cannot do. Maybe you are better with your hands than your feet or vice versa. Perhaps you like stand up over ground work or maybe the other way around. The difference in distance from your opponent may also appeal to you. Maybe you like more circular movements compared to direct ones.
All of these things will make up what you like or want in a martial art. Everyone starts somewhere and then they decide after doing it for a while if they want to stick with it or move to a different art. In order to answer the question of which is the best martial art, you have to practice them for yourself.
This same philosophy holds true when it comes to guns. Which gun is the best one for defense? The one you currently have. Start somewhere and then study it and understand it completely. Once you have done that, then you can make up your mind as to what is the best martial art for you.
There will never be one martial art that stands above the rest. It all depends on your body and what martial art fits you the best. Good luck and keep practicing!