There are different ways to measure "tough". Cliche as it may be, I doubt Dominick Cruz would handle childbirth well, and there is also an entire different varient of toughness in mental toughness. UFC fighters are tremendous athletes that take a beating, but to universally declair them the toughest is arrogant.
Is there a true way to measure toughness in the fighting world? How would you compare a Sumo Wrestler to a UFC fighter? I don't think there is a true way to measure which one is fighting harder or which one is more active. I think that UFC fighters are known for being tough to fight, but I also know that there are too many types of fighting to be able to measure which one is which.
Fighting for money in a controlled arena with a fixed set of rules and a referee who can stop the fight at any time does not make one tough. Although they get beaten up quite badly in their matches, military soldiers face even greater danger when in the field and do not have the advantage of a controlled environment or a referee to stop the war-- The men and women who fight for our country are the toughest in the world.
I believe that UFC's fighters are excellent athletes. However, there is such a hype around the whole UFC circus, and so much money is involved. I wonder if those athletes could still make such comebacks, had it not been for the immense financial cushion they all have, which gives them access to the best professionals and facilities.