I would say pac. Belt holder at 112 - 154 and recognized No1 in many of they weight classes takes it for me.
Think France's "Orchard Man," Georges Carpentier, went from 118, through every weight class, to face Jack Dempsey for the heavyweight title.
Lest we forget Ted Kid Lewis, who fought from flyweight to MW and up to heavyweights. Today he is largely forgotten, but he is Britain's best P4P fighter ever...The "crashem. bashem " Ted Kid Lewis who had 282 pro bouts...
Pacquiao fought his first fight at 106 when he was a starving 17 year old, but his size suggests that he should be a natural lightweight. He's no smaller than Duran, Whitaker, or Pryor in terms of height and reach but Duran went up to super middleweight and had wins whereas Whitaker and Pryor went up to junior middle and Pacquiao has only gone up to full welter at 147. Henry Armstrong is a full inch shorter than those guys and still managed to fight his way up from featherweight to light middleweight (since Ceferino Garcia only weighed 153 pounds when he fought him). Beanpoles like Alexis Arguello and Tommy Hearns I wouldn't give the same kind of credit for weight climbing. They were physically big enough to fight several weight classes above where they did, so should really be considered great weight cutters. Same goes for Robinson fighting at lightweight and welterweight. How often did he ever fight someone taller with more reach than him there? He's got super-middle size bones and muscles and he's just murdering little guys. Of course, Bob Foster hit like a truck at light-heavyweight. He's 6'3" and should have been a heavyweight. There are a couple of guys like that. A guy like Mickey Walker 5'7" with 67" reach fighting Jack Sharkey who's 6' 72" reach or Arthur De Kuh topping 6'3" and 223 pounds, that's the real deal in terms of weight climbing.