Jimmy Young. Fought at least evenly with Ali and Norton. Could have easily gotten the decision or at least a draw. Dropped Foreman and won the decision. Easily decisioned Lyle twice.
An excellent shout, Young is seriously slept on and was a very talented boxer. Beat Lyle twice and Foreman on points.
The flipside to this question is just as interesting... Who's the worst heavyweight to win a proper belt? Or the worst established (ie: non fluke and immediately lost it) heavyweight champion with a 10+ fight reign?
I consider him a legit champion but I know he technically didn’t win it in the ring. Even we are discounting that then I have to agree with you.