TV Sports Boxing (AKA: ABC Wide World of Sports boxing for the PC and Evander Holyfield Real Deal Boxing for the genesis) is, IMHO, the greatest boxing game ever made. It had the best career mode by far, although the genesis version was too watered down compared to the other two. You can still download this game from Cinemaware from their classic vault:
http://www.cinemaware.com/clsgame_tvboxing.asp
(I wish other game companies take note of what cinemaware is doing for the fans of their old games!)
You will either need to get DOSBox (DOS emulator) for the PC version or WinUAE to emulate the Amiga version(very complicated.)
A close second goes to 4D Boxing, which only needed a better career mode to take a comanding lead for greatest boxing game ever.
-BT