Aha! The formless form! The style that is not a style!
This thread reminds me of something I read some time ago. It was to the effect of the most dangerous swordsman to face, aside from a master, was one who had never held a sword before. The rank novice could be incredibly reckless and unpredictable, and while being very dangerous to themselves, could also be a hazard to a trained swordsman.
As far as actual styles go I'm not sure that there is a most dangerous one. Really it is the individual, their level of and commitment to training, innate factors, and their level of disregard for their opponent.