While there are literally hundreds of styles of martial arts around the world far too many focus just on style and the sports elements and are not designed for actual combat and use for taking out a real actor in a real life situation. Too many of these schools teach theory and moves that will take far too long to actually kill an opponent or be effective against someone intent on killing you. You may be surprised to find that most of these you will not see commonly used in the UFC, basically as the techniques taught are banned and are specifically meant to kill.
MCMAP is the United States 'Marine Corp Martial Arts Program'. This battle tested form of martial arts has been developed and used in the world's most dangerous war zones and has been proven to be extremely effective especially against armed attackers or when you run out of bullets yourself. A number of UFC champs have tried seeking out MCMAP training, although it's techniques are reliably well guarded and are no doubt not allowed in the octagon. Unfortunately if you want to train in MCMAP you may have to sign up for the US Marines for 4 years, though you will absolutely be assured to come out a fit killing machine, well without it kills you first.
2. Muay Thai
Muay Thai kickboxing is certainly one of the most brutal forms of martial arts in existence. Not many rules here and no points deducted foritting your opponent as hard as you possibly can. Definitely not for those afraid of pain. Muay Thai's use of elbows and knees can prove to be extremely deadly.
3. Kyokushin Kaikan
Kyokushin Kaikan, Karate is certainly one of the toughest base martial arts around. Schools offering Kyokushin Kaikan, Karate are as far from a McDojo as you can get. No easy belt gradations here. Kyokushin Kaikan is known for it's full contact sparring with bare knuckles and it's 50 & 100 man fights in order to pass graduations.
4. Krav Maga
Krav Maga is the Israeli special forces form of tactical hand-to-hand combat. Training in Krav Maga will absolutely teach you how to kill with your bare hands in seconds even when up against an opponent armed with a knife or gun. Krav focuses exclusively on hitting weak points such as the throat, spell and temples. There is pretty much nothing learned in Krav that would be allowed in the UFC, far too deadly!