Determining their prime is what is difficult. Is it a single fight? A whole weight class? A one year span? A varying span of years? And most importantly are we talking about who was better H2H and skill wise during their prime or who accomplished more in their prime? If I go by the above standards of a) varying span of years and b) who accomplished more, I might arrange them like this (although an hour from now it might change): 1. Pacquiao- 2006-2009; Erik Morales II, Oscar Larios, Erik Morales III, Jorge Solis, Marco Antonio Barrera II, Juan Manuel Marquez II, David Diaz, Oscar de la Hoya, Ricky Hatton, Miguel Cotto 2. Barrera- 2000-2003; Luiz Freitas, Jose Valbueno, Jesus Salud, Naseem Hamed, Enrique Sanchez, Erik Morales II, Johnny Tapia, Kevin Kelley 3. Morales- 1997-2000; Daniel Zaragoza, John Lowey, Remigio Molina, Jose Luis Bueno, Junior Jones, Angel Chacon, Juan Carlos Ramirez, Reynante Jamili, Wayne McCullough, Marco Antonio Barrera, Mike Juarez, Kevin Kelley 4. Marquez- 2006-2009; Terdsak Kokietgym, Jimrex Jaca, Marco Antonio Barrera, Rocky Juarez, Manny Pacquiao II, Joel Casamayor, Juan Diaz Cruising boxrec to determine this is sort of fun... One interesting note is that JMM has an insanely tough resume to determine the "prime" or "best" span of years. Arguments could be made for a few of them... It's sad to look at the twilight of their careers for Barrera post-Pacquiao II and Morales post-Pacquiao I.
Pacquiao should be 1 due the names he defeated while in his prime, Marquez should be 4 because he was pretty much frozen out in his prime years and didn't get the chance to stamp his mark. Morales/MAB are about the same maybe i'd give a slight edge to Morales considered MAB has KO loss in his prime.
Marquez never got the acclaim he deserved in the early 2000's and couldn't get the big time fights with other big name fighters in and around his division. His resume is pretty dam impressive. Seeing him go overseas and take on Chris John in what I thought was close to a 50/50 fight and from that point forward the resume he's built for himself is nothing short of impressive. Depends also on your viepoint on how Maruqez did with the Pacman in his four fights. Personally I had JMM beating him in all 4 fights, including that 1st one where he was knocked down several times in the 1st round. I have it: Marquez Pacman Morales MAB I don't like putting MAB all the way down on this list, but H2H he's only had success against Morales. In his prime he lost decisively to Junior jones (2 x's I believe,) but he did beat a prime and undefeated Hamed. I can't put MAB over Pacman because Pacman had his way against him H2H (pummeled him in their last fight,) and the last 10 years or so, aside from losing to JMM, Pacman has put together an ATG resume. All 4 great fighters and first ballot HOF'ers, but 2 of them clearly stand out as ATG's.
I'm tempted to say Pac, but then he got beat by a Morales who was already well past his best and never really got the better of Marquez in 4 tries. Looking at how they did against eachother overall though, he still comes out ahaed by a nose. He's the only one who (officially) holds a winning record against all the other 3, and his overall resumé and accomplishments are vast. On a side note, i give Pacquiao a lot more credit for what he accomplished in the lower weightclasses than for anything he did above 135, where it's a lot of smoke and mirrors.