Depends upon what the streets are like where you live. If attackers tend to be one unarmed guy in your neck of the woods sport BJJ is great - competition mimics the adrenal surges and intensity of a real fight.
But where I live the average attack (according to the police) is two to five guys with knives and often firearms. In that case the best self-defense art is the 110 m hurdles (better than pure sprinting because you're used to uneven ground and jumping over things like fences, cars etc). Failing that, something where you use a weapon.
Even an untrained human with a knife or stick is a much better fighter than all but the very elite unarmed fighters. I've taken to carrying a very heavy walking stick around, looks harmless, but its five pounds of solid oak, with a metal ball at the end. I'd use it over BJJ, judo, boxing, MMA, or any other unarmed art.
And as the Dog Brothers say, never bring a knife to a stick fight
Of course, against five guys I'd still rather be great at the 110 m hurdles, and if they have firearms its even worse (though a lot of guys who carry handguns can't hit anything more than 15 feet away, so running actually is an option).