Normalized for speed and strength, making this only about boxing skill, the list above me is a decent one (minus Cage). Unfortunately, the true answer might just be the most skilled MAers. True, it's not boxing-specific with most of them. But comics are their own beast; the general skill levels shown by dozens of martial artists means that they'd transition well to pretty much any style, boxing included. So Karate Kid is a good default answer. And #'s 2-5 might just be similarly haxx MAers.
By the way didn't know Cage actually had boxing skills, I just thought he was an experienced and skilled street fighter, then Iron Fist taught him some stuff.
__________________ Watch what people are cynical about, and one can often discover what they lack.
- General George Patton Jr