I can't pick one but if I had to it would be Leonard vs. Hearns I. A legendary comeback, the "you're blowin' it" speech from Dundee. A true classic that showed the best of both men. That's probably my favorite. Then Hearns vs. Hagler for obvious reasons, just a great fight that gave Hagler his just due. And finally Leonard Duran I, just a great fight in general and the best win of Duran's career and one of the best wins in boxing history.
Oh man, how can I even try to choose? All of the fights apart from Leonard - Duran III and Hagler - Duran are just real great. If I HAVE to choose, I'll go with the original fight. You just can't top The Brawl in Montreal.
I loved them all. They were all part of the same thing and built up the previous one. It worked out perfectly. Ray learned from Duran and beat Hearns, who learned from Ray and beat Wilfred and Roberto. Marvin beats Tommy after Duran and Ray beats Marvin... The whole fight list was needed. And the personalities. Something lacking now in boxing. Who knows why.