TOUGH QUESTION!!!! But for the overall completeness of the opponent,(Leonard was a better all around fighter than Ali) factoring in the winner was naturally smaller coming up to the opponents natural weight class. Duran vs Leonard in Montreal was the greater victory....
I voted Duran but obviously Frazier's performance against Ali in 1971 was also great. But Duran beating Leonard, we're talking about a former lightweight world champion who wins the welterweight world title by beating the best boxer in the category (and a great champion). Then I think that in that match Leonard totally got his tactics wrong (which in fact were completely different in the rematch) but Duran's was a great victory, and certainly, a great performance.
Both were two of the best wins in boxing history but I think Duran was a slightly bit more impressive overall just because it was someone moving up and fighting a bigger ATG rather then two ATG at the same weight class
Both Duran and Frazier fought like absolute mad men and put on arguably their best performance in their career as well as well as one of the best victories in boxing. If I had to choose which was the better performance I would say Duran looked more complete beating Leonard. Not only did he outfought Leonard but he also outboxed him at times and was hit less.
I voted for Duran primarily on the basis of Leonard being either @ or much closer to his peak than a post-exile Ali was to his.