The HBO choices: Pacquiao I (2004), Barrera (2007), Pacquiao II (2008), Diaz I (2009), Mayweather (2009), Katsidis (2010). Fine choices. You have to include the Mayweather fight for significance. As for why they didn't have the Casamayor (2008) fight in here, considering that's where he won the lightweight championship, recall that that fight was merely distributed by HBO, not produced by them. That's Golden Boy-owned, I believe.
As I mentioned earlier, I rate the second Pacquiao vs Marquez fight as better than the first, and I think JMM had a better performance in that one, too. It's not just about the fact that he didn't get knocked down three times (he did get floored hard one time, in the third round of the rematch) -- it's that Pacquiao had gotten better, and Marquez was once again there with him, step-for-step, showing that he'd also become a better fighter. I also just feel it's a better action fight than the first. Marquez was easier able to neutralize one-hand Manny in the first fight than he was the second time around.
And I also still feel that Barrera vs Marquez is a really underrated fight. That was a hell of a battle.