What is the most dangerous martial art?
The one being used by an unskilled, un-trained individual. They will hurt more people, including themselves, then someone that knows what they are doing.
The most recent "established" martial art would have to be mixed martial arts or MMA. Unfortunately there is some debate to whether or not this an original martial art because it's just an amalgamation of several techniques from other martial arts. It combines everything from Judo to Greco-Roman Wrestling.