You know, it's beyond a cliche at this point but Bruce Lee really for all his arrogance was quite brilliant. Dana White (UFC CEO) calls him the modern day father of MMA for not believing in styles. Chinese styles versus Korean styles versus Japanese versus Western styles. He just took all of it applied, and to me this is the most sensible thing that has been said about the subject.
For all the good things Martial Arts bring it's still grounded in a lot of cultist behavior. Not all cultist behavior is evil or out of malice, but it's removed from rational thinking taking a biased fixed stance. So cross-training.
MMA has been fascinating when watching a grappler specialist versus a striker specialist or a take-down specialist. Power vs speed, flexibility versus endurance, iron chin versus mental toughness. There is so many amazing factors that the whole idea of subscribing to a "best" system, is not something I believe in anymore.
So cross-train. If you can join a gym with multiple disciplines that should be great for you. I would suggest for the first years perhaps only dabbling in one (possibly two) things to get a good basic understanding.
But with such a high dropout rate, the chance of people sticking with it is low.
At my fight gym, every day we see new young men come in. Buff, tattooed and with military cut hair, and 90% of them give up within the first month. The people who seem to stick are the people who look anything but fighters. the nerds, the casuals, the out-of-shape person transforming their lives. It seems that having the same skill for committing to something in other aspects of life carries over to the martial arts, as this is yet another thing that requires 10,000 hours of mastery.
So if my first suggestion to you is to cross train, my second suggestion to you is to put in the 10,000 hours to get good, and to stick with something for that amount of time you will need to have fun. Even if you take a discipline that some people feel has a more limited application in self-defense, I think you putting that many more hours into it will outshine your grinding in a more useful, but less fun discipline. It's simply too much work and sacrifice if you are not having fun.
Boxing, Judo, BJJ, Thai Boxing, Greco-Roman Wrestling, Sambo. You can't go wrong with any of these.
I do not think any of these disciplines will guarantee victory. The name of a street fight is that its as likely/unlikely to result in victory as the terms allow. For one it depends on the strength and martial ability of the person(s) your fighting. Secondly I have not personally seen any art that really can guarantee success against multiple assailants, and it is my understanding that most street fights involve more than two people - Usually a group of people jumping a single or fewer persons.
Then there is the situation of weapons. Krav Maga is the only one that is going to teach you how to disarm, though some arts like the stick-based Filipino arts can teach certain knives disarmament techniques as well.
Also, depending on country, even defending yourself might result in a law suit under the guise of excessive force. A friend of mine who is a lawyer have told me about cases were an aggressor sued a defender under the pretense that he was using martial arts as an excessive way which caused unnatural injuries even in the face of self-defense. Obviously it's a joke, but it is something to keep in mind and simply another reason to run instead of fighting if you can avoid it. You might actually go to jail, or get an assault charge even if you did nothing wrong but defend yourself. "Martial Arts at the end of the day means, honestly expressing yourself" is another great quote from Bruce Lee, and I think it holds true.
When I spar with someone I feel I get to know a person in ways words can't express. You get to see a lot of a persons character based on how they fight. Are they aggressive, passive? Are they are sore loser or a good winner? do they have honor and humility or are they brass and arrogant? you get to see their sense of competition, inner drive and how they work under intense physical stress.
It's a whole different way at looking at human beings. We give so much precedent to how we carry ourselves verbally. your out of the game if you can't verbally explain yourself, but in fighting, like other arts you get the see something truthful about a person and their character. That is why you must find the martial art that speaks to your temperament, philosophy and idea. Don't fear so much that you are picking the wrong thing. As corny as it is it really is a way of life. You don't have to sit under the bodhi tree tree meditating until your levitate, I just think there is no way that this won't change you.
The whole way I look at strength, at self-reliance and why bad things happens to me, got turned upside down when I started martial arts. Every time I went to class I failed. The first couple of years was just a blur of getting submitted, dominated, bruises and injuries with little gain. But then I realized that all this was not failures. All these things were making me stronger and one day I was the one who could defend myself, and had gotten my own personal philosophy and martial arts game and style, but perhaps most importantly, being able to carry it on. That's ultimately what I think it's all about. I am not sure if a lot of this rambling has much to do with the idea of what is the best self-defense. I think the best self-defense is to cross train, and I think the best self-defense is to put in the amount of work it will take to achieve really good results and due to this you have to explore all the options for myself.
I think in time that you will find that it is actually a blessing. that the answer is not fixed, it's simply your discovery of it that is the answer.