Probably Miguel Cotto for what he's accomplished. There's no way to know with any certainty, but i think Andre Ward will rise to the top of the list in a couple years.
A fighter gets "exposed" only when we learn something about him we did not already know. For example, we did not know that a tough but crude brawler could walk Andy Lee down and beat hell out of him. Brian Vera exposed him. Cotto lost fights to two of the best guys around his weight. He was not "exposed." He merely lost to two other elites. Your use of the word was inappropriate.