Right answer is probably some old timer where the size diffrence would not be quite as big - but when He was boxing, I thought He had a chance of becoming a middleweight "champion" against Sebastian Sylvester, holding one of the titles at a time.
I have two names of world middleweight champions that he would have definitely beaten Pacquiao, De La Hoya and Cotto. Then I think he would have had a good chance against Javier Castillejo too.
I'm not sure but he was certainly capable of being a MW champ. Like Ike said he beat 2 MW champs. When Manny was going up to 154 it was sort of a lull period for MW and there weren't many fights to be made there compared to WW. That wasn't a bridge too far for him there just really wasn't a reason to cross it. This was the time where the commentators spent the whole Manny fight openly sulking he wasn't fighting Floyd instead. People were really pigeonholed at the time they wanted Manny to fight Floyd.
I don't think He'd beat them at Middleweight, though. With his body frame, He likely wasn't able to go beyond his 150 max fighting weight effectively, while Cotto and Oscar still carried the extra weight reasonably well and would have an advantage over Pacman at a higher limit.
Roberto Duran. I don't rank the guy at 160, but he'd beat GGG and Jermain Taylor according to Classic.
I think Pacquiao could beat a guy like Daniel Geale or Anthony Mundine. I think he would even be favored over guys like Adames, or, anybody in the top 10 today except Janibek. The once great Middleweight division is flat out garbage lately.
I understand your reasoning but I don't think a few extra pounds would have affected Pacquiao so much that he would have lost to them. De La Hoya was no good at middleweight, he won the world title against Sturm in a match that I thought he clearly lost and then he was knocked out by Hopkins. Cotto was a little better than the Golden Boy but he was a small middleweight and I think, without wanting to take credit from the Puerto Rican champion, that if he had met a Martinez with healthy knees the match would have ended differently. I doubt that a few extra pounds would have changed the match that Pacquiao and Cotto fought at welterweight much. I think instead that the De La Hoya seen against Ike Quartey at welterweight would have been a difficult opponent for Pac Man.
You're probably right about Cotto. He was never a middleweight anyway.. Only beat crippled Sergio and weight-drained Geale there and never weighted more than 155 pounds. With Oscar, I don't know - it has to be said that the version that Pacquiao beat was completely dead at the weight. Even the version that - I agree - lost to Sturm was significantly better and would've been much more difficult challenge.
I'm surprised nobody has mentioned Manny's compatriot, Ceferino Garcia. He wasn't the greatest middleweight, but pretty darn good, and Manny could beat him. I would favor Manny's chances against someone like Jake Lamotta and Rocky Graziano.
He might beat Vito Antuofermo on cuts. Vito started bleeding at the weigh-in. With no such cuts scenario, I’d say maybe the best Pacquaio would be able to beat would be Joey Giardello — not a big puncher, not particularly fast of hand nor foot, often got himself off-balance when punching. I could see Pac outworking him for a decision, although he certainly never hurts Joey.