probably MMA, if you are not steering towards traditional. MMA is a combination of ground fighting, including Jiu Jitsu, and Judo and stand up figting, including Karate, Muay Thai kickboxing and many other techniques. Taking Mixed martial arts will teach you the moves and how to defend yourself from every angle and every situation, whether on the ground or standing up.
If you are leaning for a more traditional aspect, I recommend
Goju Ryu Karate.
Brazillian Jiu-jitsu.
Most people don't know what to do when they have to fight on the ground, and if you know Brazillian Jiu-jitsu you can easily dispatch your opponent.
krav maga. if you are looking for hardcore self defense and a way to kill someone. this is not a MARTIAL ART (as it is not an art form as is tae-kwon-do)
Israeli army, American military and special forces worldwide all use krav-maga. as do most police agencies and private military contractors
krav maga by far. its used by israeli soldiers (and jason bourne) and its purpose is to [disarm] and disable your opponent in the most efficient and devastating ways. it is also simple to learn cause krav maga relies on using the body's natural reactions. O and there are also no rules because it is technically not a martial art, it is a form of self defense, meaning that nutshots and other nasty moves are allowed. it is also meant to be used by anybody, including women and children. It is the simplest, easiest and most effective fighting style on the planet
By far TaeKwon-Do i have my black belt and the course has come in handy in some Very dangerous situations, it is a bit difficult to get the hang of at first tho.
it's hard to say one is the best. I like Krav Maga as it is very practical and deathly quick, Jeet Kune Do is easily the most versatile, and Traditional Jiu Jitsu is quite the effective practice.
My favourite though is still ninjitsu. Though it is not something you can just pick up as a hobby but more of an actual lifestyle of learning/studying and practicing.