there is a lot of speculation in this thread, but i've actually trained in most of the styles mentioned. i also compete competitively in submission wrestling and several of my good friends are pro and amateur mixed martial arts fighters (a.k.a. vale tudo, no holds barred, ultimate fighting, cage fighting, etc).
let me tell you first off, that physical conditioning is the biggest factor in fighting. if you are strong, powerful and athletic, you will have an advantage in a fight, even over a moderately skilled (but weak) opponent.
second of all, it doesn't matter what style you study, as long as they train full-contact with a 100% resisting opponent. what styles do this? boxing, wrestling, judo, bjj, mma, muay thai. it's no surprise that these styles are also considered the most effective martial arts. (notice i did not include krav maga and systema and other "lethal" top-secret special ops military styles)
third, to be effective in a hand-to-hand street fight, you need to cover all the ranges of fighting--striking, clinch and ground. for striking, boxing and muay thai have the longest history of proven effectiveness. for clinch, greco-roman and judo are two of the most effective in this range. and for ground, BJJ is by far the most effective. if you become proficient in all three ranges, you will be fine in a hand-to-hand confrontation.
lastly, and maybe the most important thing, no art will give you the magical skill of defending against a knife or a gun. any martial art that claims this is selling you a fantasy.
the best self-defense is awareness and avoiding dangerous situations. you can avoid 99% of dangerous confrontations.