Technically, what we usually and often see right now are martial arts through sports. It's not enough basis to conclude that such is the "most dangerous martial art". Martial arts derived it's name from the Roman god Mars, the god of war, thus martial arts means the "art of fighting". And it's so ironic that we decide martial arts merely by basing most of it from sports which are bound by rules.
Each and every discipline has their own strengths and flaws. Some are designed for multiple attacks, some are good in ground fighting, and there are even some which are good just to create distance for one to flee. Either way, any discipline is effective if it could preserve one's life, and it leads to the practitioner's experience and not the martial art discipline itself.
There's no point of arguing as to what martial art discipline is the best or should we say the most "dangerous" or effective since different practitioners from different disciplines would always say that theirs is the best.