To the best of anybody's knowledge, no randomized controlled trials have been done to determine, say, whether karate is more deadly than krav maga. This may be because scientific studies that require people getting offed are generally frowned upon.
It's worth noting that most of the premiere butt-kicking schools that get extra points for lethalness begin with the local street-fighting customs of a given region and build out from there, so maybe you want to go a few rounds with the neighborhood bully to work on your chops. By the way, you are going to use your powers for good, aren't you?
Sambo is not a Brazilian dance but a real hard-edged fighting style that arose out of post-Bolshevik Russia when people had a lot to fight about. The word "Sambo" is actually an acronym for "Self-defense without weapons." It's said to incorporate all kinds of aboriginal fighting styles from all around the world into one turf-grabbing onslaught of aggression.
Watch a Sambo match and it's like you're watching a kid's playground fight gone nuclear: their dukes are up like boxers, then they start kicking viciously at each other's chins, then somebody's getting their head ground down into the pavement. And that's Sports Sambo. Combat Sambo takes no prisoners.
Krav Maga is a melange of several sophisticated and brutal Asian martial arts techniques, but hand-to-hand combat always seems to have its own local color. Now used by the Israeli Defense Forces who have continued to develop it, Krav Maga arose from the street-fighting experience of Hungarian-Israeli martial artist Imi Lichtenfeld, a resistance fighter against fascism during the 1930s in the Jewish quarter of Bratislava, Cezechoslovakia.
Krav Maga takes no prisoners, nor does it require stupid sacrifices. Krav Maga's philosophy is to devote equal effort to offense and defense. It's all about, "whatever it takes" which also means you can turn tail and run if the situation calls for it.
Thailand is the only country in South Asia that has never been colonized. You can call that good management, but it also helps if you know how to fight. Muay Thai -- known better as Thai kickboxing -- is based on "the art of the eight limbs." So there's a strike for every "limb" -- which includes your elbows and knees. The strikes are representative of the eight ancient weapons of war: punches, elbows, swords, hammers, kicks... well, you get the idea.
Tai Chi Chuan
Tai Chi may seem like a nice mellow exercise for older people. If you live in a large urban area with a significant Chinese population, you may in fact see the seniors up and at 'em at the crack of dawn, practicing the ancient martial arts form.
Fluid, graceful and not very challenging, you may think. Wrong. Performed at peak technique, Tai Chi is regarded by martial experts as by far the deadliest of the martial arts. Tai Chi is about understanding that everything is either energy or space and that matter is illusion. We're talking "Crouching Tiger, Hidden Dragon" stuff, people.