Below is a few points for speculation and discussion from my own eleven years of practice, conflicts, and research. For background information, I practice a mixed style of Krav Maga, Muay Thai, and Tae Kwon Do. And please, don't take all the statements as completely correct, since there are many outliers in life.
I agree with the majority on this issue. Krav Maga is probably one of the most effective because you start out with self-defense and it is a major focus in the training. The teach you how to build on your bodies natural reactions in a fight (i.e. covering your head)and to aim towards weakspots and "cheap shots" like the groin or eye-gouging. If you had to learn how to defend yourself in a week or two, this would be my recommendation since they develop the basics quickly and effectively.
Unfortunately many Tae Kwon Do schools, from my experience, have a problem with self-defense. A lot of schools I've seen are focused on the sport application, some of which do not even teach proper blocking! The martial art is effective in self-defense if it is taught properly, kicks to inside of the thigh to numb the leg (hitting the nest of nerves), taking out the knee, crushing the shin, kicking the kidneys, a surpise kick to the solar plexus, and even imploding the rips and dislocating the jaw/crushing the skull, etc. The power comes not from tensing the muscles through the motions, but from a fast strike where the entire limb only tense on contact. It is more of a soft-hard style than a completely hard or soft Qi Gong. Which makes since because legs are slower than arms hahahaha. Many modern combat styles like krav maga and USMC style focus on the soft hard because of its quick and powerful strikes that are great for vital and weakspots of the body.
It is important that you state at the practice area that you are taking the martial art for self protection. Most will not be as fast as krav maga though, especially kicking arts since it takes awhile to train the legs. However a capable instructor, even in the kicking arts, will teach a person basic strikes (arms and maybe low kicks), blocks, disarm/basic holds, and mock fighting for application. Yet again, it is unlikely they will teach you to fight tooth and nail like Krav Maga, but you will be more prepared with some training rather than none.
Aikido is also a good alternative to Krav Maga if you cannot find a school or prefer a more passive approach. It is a sof Qi Gong style that focuses on redirection of an opponents energy and controlling it. Interesting internal energy theories in the art, you can also ask to focus on self defense, and it is easy on the bones and joints for elderly or more fragile folk.
Martial artists must continually seek to understand their weaknesses and improve upon them, whether it be throws, ground combat, etc. Krav Maga is great for learning quickly on how to defend oneself, but the true ability of self defense is in the mindset. You must be aggressive, but not blinded by it. You should acknowledge that you will be hit, you will be stabbed, and you will be shot so if those events do occur your mind/body has less of a chance of freezing allowing you to continue/initiate action. It is not the elimination of fear, but the harnessing and using of it. Fear is a great motivator hahahahaha.
Edit: To the post above me, thank you for reminding me. You don't always have to fight, running away or conceding the ego is a nice alternative to getting hurt. There is a nice story of a capoeira artist who was confronted by a man with a knife. Instead of fighting, the artist ran away into a crowd of people while being pursued. The artist fell to the ground and screamed "Help help, he's trying to kill me" while pointing at the man. The man freaked out as the people and police officer all stared at him and he ran away. Funny anecdote for me anyways
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