Well, the problem with comparing the UFC to real-world martial arts is that they have different goals.
In the UFC, the goal is to win without inflicting lasting damage on your opponent. As such there are rules and structure in place. As far as organized fighting goes, I think top-end MMA fighters would tune anyone operating in any style. Modern MMA is as close to perfect as a sportive fighting style can be.
In the real-world, however, the objectives are different. Lasting damage or even death can be a goal, disabling an opponent in order to escape can be a goal. I think that in an actual street-fight a trained MMA fighter's training could work against them, blinding them to possible avenues of attack, or blunting their force. Imagine being in a street fight and someone slaps a kimura on you and you just tap him on the arm three times and he lets go as an automatic response. Then you kick him in the jewels and run away.
As far as real-world applications go, I have no idea which martial art would prove the most effective, although this one looked pretty effing awesome in
Dark Knight:
Keysi