Kina Mutai would be up there IMO (basically the art of latching onto your opponent and biting/tearing their face/throat off).
Muay Boran (at least in it's original form) would be up there as well (especially when they used to actually coat their hands with broken glass and such).
Ninpo/Ninjitsu was all about killing/assassinating someone quickly and silently through the use of poisons, stealth, various projectiles, etc...
As far as which arts would allow you to drop an opponent the quickest, that would probably include a pretty long list of arts.
Boxing, kickboxing, Muay Thai, Muay Boran, various karate styles, various gung fu styles, etc... will all give you tools to be able to KO someone.
Jiu-jitsu, judo, sambo, catch wrestling, and other various grappling arts will give you tools to be able to control (and drop in some cases) an opponent very quickly as well. In a life or death situation these controlling holds could be used to do serious damage to the opponent (even fatal in some cases).
As far as what's the best for self defense, you'll get a lot of opinions on this question. Everyone has their preferences/prejudices and it's really hard to say that any one art is unquestioningly the "best" when it comes to self defense.
IMO your best bet is to find a school that teaches multiple arts, that cover the full spectrum of the combative skills, blended together into a cohesive whole and who's focus is on real world self defense.
It can be tough to find such a school though, look around and see what potential schools you have to choose from. Try out a couple classes at the schools that interest you and then make a decision as to which school best fits your desired goals.
You know...for a person with no martial art or self defense background,I really think that learning basic boxing skills should be a priority before anything else....except common sense. A good boxing skill set will carry over and help tremendously in whatever martial arts or self-defense concepts a person chooses.
For self defense in a life or death situation, arts which focus on gross motor movements seem to be the best. When your adrenaline is at 120% it may be difficult to achieve the "heaven and earth breathing throw" of Aikido
Yeah, I should have made it clearer. I meant speed with which you can drop an opponent with the least effort on your part. Krav Maga really stands out from what limited experience I have of seeing it in action.
Muay boran is very efficient at taking down an opponent easily. Of course in all martial arts, like sentoguy mentioned, once you are skilled at it, you can quickly take down your opponent. But all things being equal, it is faster than say a soft art, like aikido.
And it is brutal. Not brutal as in drawing blood, ripping, etc, but as in quickly dispatching your opponent.
Part of the reason that I became disenchanted with Okinawan karate was my learning of the basic skills in boxing.
It's hard to listen to people tell you how you're supposed to punch when they're not teaching you the footwork that adds all the power, don't teach angles, and don't teach you to slip or bob and weave.
In self defense situations where one hard shot to your jaw is going to end your run, you better be able to see that shit coming, move, then bang back and take off.
I agree with everyone who says get into boxing. It's a great foundation. If you're trying to find that martial art that is better than them all, you won't find it. All of them have their uses. If I were to recommend a martial art though, I'd go with Kyokushin karate, it's founder was Mas Oyama and it's all full contact.
If you are so determined, you could always give the 3 year live in program a try. Go to budokaratehouse.com and click on the residential program. If you are not interested in doing the live in thing, you could always see if there's dojos in your area.