I believe that when people ask this question, that they are really asking "What is the perfect martial arts system that will work 100% of the time?" Here is the answer to that question: it doesn't exist.
No martial art is perfect and none of them will work 100% of the time. Even a Kravist can be beaten with the right technique that is executed at the right time with the right mindset. So instead of searching for some mythical, all-powerful Shaolin magic wand, you need to ask this question: which martial art best suits my needs? The only way to discover the answer is to try out multiple systems.
If you've never trained in the martial arts before, then I personally recommend that you study Krav Maga for at least three months first. It's practical, it's efficient, and covers all of the basic aspects of self-defense. Then, I would find the area that you are the weakest in and cross-train in a martial art that specializes in it. For example, if you need to work on your striking, study karate or kung fu. If your grappling needs improvement, learn Brazilian jiu-jitsu or judo. If you need to learn how to fight in the clinch, study Muay Thai. Obviously, there are many valuable martial arts besides these which you can also learn. For example, MMA has proven that amateur and Greco-Roman wrestling are severely underrated martial arts styles. The key thing is to find those that are best for you.
If you are already a black belt in a traditional martial art, you should still study Krav Maga. Why? Because it teaches you how to best deal with modern weapons. No, I'm not telling you to go sell your katanas on Craigslist. Training with traditional martial arts weapons is still a great way to exercise, make friends, and have fun. I'm just telling you that the odds of a street thug or rapist attacking you with a pair of nunchucks are slim to none. You may be able to knock a walnut off someone's head with a shuriken while blindfolded from twenty yards away, but that won't help you if a gangster takes a .45 caliber and aims it at your forehead.
Also, if you have a black belt in a striking martial art, cross-train in a grappling martial art and vice-versa. All of the spinning kicks in the world won't help you if you are thrown on your back and don't know what to do. Likewise, you may be able to flip a linebacker by only grabbing his pinkie, but that won't do you any good if you get knocked out with a front kick because you lunged at a Tae Kwon Do champion.
If you do MMA, you'll have enough exposure to both striking and grappling techniques to be a well-rounded fighter. In this case, take Krav Maga to learn how to deal with weapons and multiple opponents. Doing Krav Maga will also remind you that in the real world, there are no referees, no mats, and no weight classes. Not to mention that there is nothing keeping your opponent from fighting dirty (i.e. biting your leg to get out of your triangle choke). Will a traditional martial art still benefit you? Absolutely. For example, forms will help with your flexibility, breathing, and fluidity of movement. However, your coaches should have already picked out the most practical techniques from each system for you to learn, so it's not absolutely necessary.
The key thing is to find the martial art that best meets your self-defense needs. Whether it is pencak silat, capoeira, modern pankration, or any other system, it really doesn't matter. If it allows you to return home safely to your family and friends, then it is worth your time and money. Don't get hung up on whether or not one particular style is better than another, because, as Ip Man said in the movie after his namesake, "It's not about the style. It's about you."