Kostya lost because Dave Parriss allowed Hatton to constantly,and I do mean constantly,hold and hit,which is legal in MMA but not in boxing.When I hear the Joe Cortez complaints,I say look at the Tszyu-Hatton fight for a clinic on bad refereeing.
I can't think of a boxer in recent memory who built an entire career out of one good win to the extent that Hatton did. Even though Tszyu was old and inactive this win is so far ahead of any of his others that the question is pretty redundant.
Probably Kostya followed by Malignaggi, JLC, Urango and Maussa in that order. Urango could potentially move up if his career follows its current progression.
It's still his best win. Regardless of his questionable tactics or if Zoo was past it, he made the guy quit.