I have only been cognicent of boxing since the 80's so I really couldn't tell if he can be classified in the Greatest Ever category yet. But from the fighters I've seen from my generation, the Pacman is right up there with Mike Tyson, Suger Ray Leonard, Roberto Duran, Marvin Hagler and yes, the great Muhammad Ali.
Of course the criteria used will make a difference but think for a moment where Manny is at this point of his career. He has won his last four fights in different divisions against champions. He has demolished the last three with the greatest of ease. He has charisma that attracts millions who idolize him and consider him a hero. Manny "retires" champion boxers. The only fighter who he has not sent into obscurity is Juan Manuel Marquez. The only that can stop Manny for boxing immortality is politics.
Is he the greatest ever? I think he's the greatest now. The greatest maybe even the last 20 years all things considered. I think if he wins a few more against future Hall of Famers like Mayweather Jr., talk of greatest ever is fair game.