depends on the level of the assaliant.
boxe is great with striking but the moment you add kicks to the equation, and an opponent that is good with them, it gets significantly less effective
the usual answers about what is the best one it would be brazilian jiu-jitsu, since most street fights end up on the ground, BUT I'd argue it's not great for a street fight scenario for two reasons
1. in a no-rules fights the ground is not where you wanna find yourself, that's because you have no idea if the guy you are fighting has friends in proximity and in those cases, especially with alcohol flowing, human heads tend to start looking like footballs
2. since there's no referee or tapping out, once you get an hold in you either choke somebody or break their bones, and that can have some legal repercussions, even if you are the one getting attacked. You might not care about it now, but you will in the future.
unsurprisingly the best discipline to lean for self defense is...multiple disciplines.
a good striking style: muai thai is probably the best, especially for the conditioning and the tendency to use elbows and knees that are less prone to injury when hitting something hard
A good ground style: some BBJ for ground scenarios but with the idea that the ground is a place where you don't wanna stay for long
and maybe some more "no-holds-barred" systems like krav maga or generically street fighting so that you learn how to recognize scenarios, learn positioning and how to use objects around you to your advantage.
I would add they are supposed to be good for training against weapons, but I don't wanna encourage this kind of shit...the main idea against a weapon, even a small knife, is to RUN (or if you are getting robbed just give them what they want).