Did Leon even have a prime? You could say his best win was beating Ali but then he could hardly have been said to be at his best after, what only 8 fights into his career?
Michael was the better fighter. But when brothers fight each other conventional logic seems to go out the window, which is why I can't pick one.
Michael was more skilled and had the better punch so he would be the obvious favourite. Not a foregone conclusion,though. If Spinks junior fails to connect with his Jinx,Leon could outwork him enroute to a decision victory
Leon Spinks was at his best against Muhammad Ali in their first fight. That was a "great" performance. As an amateur light heavyweight, Leon Spinks was also "feared" the same way Tyson was later when he first won the heavyweight title. Leon destroyed people at light heavy. He was ferocious. I've always been a huge Michael Spinks fan. But I don't think any version of Michael Spinks beats the version of his brother who defeated Ali. I wish they had punch stats (for Ali-Spinks 1). Leon would probably hold the heavyweight record for punches thrown and landed to this day. I don't even know if the light heavyweight version of Michael Spinks beats the light heavyweight (Olympic Gold medal winner) version of Leon. Also, kids who Leon beat up used to look for Michael to beat up, so they could get some semblance of revenge. I don't think psychologically Michael Spinks could ever convince himself he could beat his older brother Leon. IMO, at "their best" ... Leon beats Michael Spinks every time. Michael might have a shot if he landed his left hook, but otherwise Leon steamrolls him. It's the same with Vitali and Wlad Klitschko or Floyd Mayweather Sr. and Roger Mayweather. Wlad has had the better record and will be remembered as a better heavyweight champ, but I think Vitali would destroy Wlad if they ever fought. Roger was a multi-division champ and as a pro had a much better career than Floyd Sr. But, Roger has always seemed to bow down to his older brother and everyone in their family just seem to take for granted that Floyd Sr. was the toughest brother. It would be hard for Michael, Wlad or Roger to ever overcome that.
I have just as much trouble visualizing the outcome of this as I would a battle between the Klitschkp bros....
Michael would destroy him. Knock him out like Holmes did, or like he did to Cooney. Leon was a mediocre fighter with a glass jaw, Michael was an ATG with a good punch. This one ends quickly.