Marvin Hagler is the answer for me. Hagler was a great fighter and a great champion. Foreman was a great fighter but quite mediocre as a champ. Not counting his one defense of the WBU title he was only 3-1 protecting his belts. His only notable defense was the Ken Norton demolition. I also think Hagler defeated more top end opponents.
Hagler by enough.. He was more consistent. George did great vs. some styles and other styles he couldn't adjust to.. Marvin always seemed to adjust.
as awesome as Foreman was, I must agree Hagler was by FAR greater so many key wins even leading up to his days as contender, three years as #1 contender, fighting at the championship level, and a notable title reign
Marvin was at least the greatest middleweight since at least Monzon. I sometimes fancy thinking his victory over Hearns was the single greatest win in the history of the division. I have to give it to Marvin.