Pacquiao - Cotto was a good action fight between two great champions, I think. Pacquiao - Marquez IV, as well.
And Gonzalez vs Estrada. :good Edit: Estrada wasn't a champion at the time, but it was still a damned good fight. Simon Brown vs Maurice Blocker warrants some mention btx two very respectable reigning titleholders at WW. Heck of a fight between two guys were best friends.
Pacquiao v Marquez and before that Morales v Barrera, before that Whitaker v Chavez, James Toney v Roy Jones Jr.
I find this topic interesting lets explore what's been mentioned so far. Marciano - Louis : Yes looks like a great fight between two great champions but not the last great fight between two champions. Pacquaio - Oscar : Pacquaio was not a champion in that weight division when the fight took place going up two weight classes or more (I can't remember) to make the fight happen. Furthermore Pacquaio was regarded as a huge underdog and everyone was voicing their opinions on how it was cowardly of Oscar to choose that opponent. Corrales - Castillo : Neither of them was regarded as a great champion when the fight took place, it also wasn't highly anticipated. It was however a great fight. Vargas - Oscar : Having performed very poorly VS Trinidad many felt that Vargas had been exposed at that point in his career. Oscar was also recovering from losses to Trinidad and Mosley so this really wasn't much of a fight expect in the minds of some California latinos. Segura vs Calderon : Don't know enough about this fight to comment on it. Viloria vs Marquez : Don't know enough about this fight to comment on it. Haye vs Klitchko : Not gonna dignify this claim with any more comments. Barrera vs Morrales : I don't know really. I always felt that Morrales was sort of overrated and overhyped. Marquez-Pacauaio : Not so sure that this one meets the criteria. Yaegashi v Ioka : Say what? Wilfredo Gomez vs Lupe Pintor : Not sure. Leonard vs Hearns : Sure. Rigondeaux vs Donaire : Not a great fight. Broner vs Demarco : He was kidding. Calzaghe vs Jones : ***** please. Vasquez vs Marquez : Hmmm? Cotto vs Mayweather : Now that was a good fight but I don't think that Cotto has ever been thought of as a great champion. Calzaghe vs Kessler : Wasn't a great fight. Kessler wasn't a champion etc. Lewis - Vitali : Wasn't a great fight but rather a one sided slaughter that left Vitaly looking like Mr Pizzaface. Vitali wasn't a champion when the fight took place. Floyd mayweather vs Marquez : Maybe. Pacquaio vs Morales : Maybe. Mosley vs Dela Hoya 1 : Yes. Pacquiao - Cotto : Maybe. Duran vs. de Jesus : Maybe. Gonzalez vs Estrada : Hmmm. Klitschko vs Povetkin : No. Jones - Toney : Great fight, great champions. Whitaker vs Chavez : Could be. I am surprised that nobody has mentioned Lennox Lewis VS Evander Holyfield I. It was a great fight between great champions.