Miguel Cotto, he gave us some of the most brutal fights in history, showing a lot of heart. Of his only 2 losses, the first is labeled as a match where he was cheated against, and now the second is also under speculation. An immense amount of heart bestowed in apparently, a very unlucky fighter.
EDIT: I should probably also add that I only became a huge Cotto fan AFTER his loss to Margario. Inspiring stuff.