In your opinion. I say it was from Lucero to Cotto as an overall fighter. Physically it was probably from Julio to Diaz though.
I'd say ODLH was start of his absolute prime. Then it continued 'til Clottey. We saw some hints of slowing down against Margarito already but that could just be the weight he fought at.
Rarely is entering or exiting prime a fight that you can just point to, usually its a gradual process.... but for me, I would say I first noted decline in Pac in the Mosley fight, and he hasn't really impressed me in any fight since (despite doing impressive things) although he did look good in the Algieri fight, and I suppose I could say he impressed me there but have to take his opposition into account there.
Cotto-ish. Though, I think the best Manny Pacquiao, tactically and physically, was the Pacquiao from Marquez 4. He was using a ton of head movement and feints that were really off-setting Marquez. More so than any fight I've seen from him. I thought he looked brilliant and the most well-rounded I've seen of him up to the KO. Extremely aggressive, yet still tactical.
When he fought Cotto. Trust me, I'm a Cotto fan lol But the guy hasn't declined much since, technically or physically. As I see it what's declined is his passion for boxing. edit: his aggressiveness has declined
His tightness has declined, his speed has declined, his pop has declined, I see plenty of decline to be honest. I was cringing in the Bradley fights and in JMM 3 on how bad Pac looked compared to a few years earlier. He did look decent in the Algieri fight but like I said sometimes your opponent can create an illusion.
2006-2009 It starts with his Morales victories in the rematches then features his climbing up 135, 140, and 147 stopping just about everyone spare for the second Marquez fight in that stretch. All of his performances were sound and great. That stretch ends with the Oscar, Hatton, and Cotto trio of decisive wins that made him a true public star... Not just on paper with the resume, but from 2006-2009 he had noticeably improved his deficiencies from what he was before then and hadn't quite began the gradual, yet at the same time sudden, decline.
So you guys really don't think it was like 2003-2009. 122-135? He had much more power and speed at those weights during that time. Take a look at that missile he sent to Lucero. That thing was shot out of a cannon.
I think it was 2008-2010, until about the Clottey fight. I don't think Pacquiao had his prime in his lower weights, since he was a much less technically and tactically versatile boxer when he faced for instance Marquez or Morales for the first time. Back than he was one of the main players around featherweight, but the magic really started when he began climbing further up. I remember not believing my eyes when he fought against De La Hoya, Hatton or Cotto.
Pac was doing the same thing at 122 that he did to those 3 fighters. Usually within the first few rounds. And at 126-130 he fought his highest opposition.