Muay Thai(sp) kickboxing and brazlian jiu jitzsu(sp) are two of the baddest MA in my opinion. Karate and Takwando(sp) are usless in my opionon. They are arts, but not useful in fighting. Put a grappler with a kung fu guy and the kung fu guy will get his ass handed to him everytime.
Muay Thai uses jabs, hooks, uppercuts, and a bad ass kick, its all mid-close range. You kick with you shins, the kick is short and extremely powerful. You ever see a kung fu guy try and do a rounhouse or some crazy kick, it takes forever. Kickboxing you are in and out quick.
Jui Jitzsu is also very bad ass. Look into the Gracey's in Brazil. My instuctor trained with them for a while in Brazil. It consists of arm locks, arm bars, chokes, ankle locks, basically submissions. All close combat very effective.
The thing is to have a good arsenal so to speak. All fights start standing up, almost all fights start on the floor. How many fights have you seen where the guys ended up on the ground rolling around. I am not talking about pressure points or grabbing someone's hand out of the air stuff. That stuff is extremely tough to do especially when your adrenaline gets going, the first thing to go is your fine motor skills.
Thing is, when most people(except wrestlers)get on the ground they panic. And the thing about jui jitzsu unlike wrestling being on your back isn't a bad thing. Most wrestlers freak when they are put on their back because they have been trained to.
Both styles are very effective and can hurt someone extremely bad. The good thing is that it teaches you control and that you can subdue someone without having to hurt them. There are holds/locks that hurts so bad they will make you cry and beg for submission, but after its let go, they are fine within a few minutes. Now if you finish the hold/lock and hyperextend an arm or leg, that takes a long time to heal.
Just my .02