I think you would have to say his first peformance against Manny Pacquiao. To regain his composure to completely outbox Manny Pacquiao for the next 11 rounds was a great achievement. That same man he countered for 11 rounds is now today regarded as the best fighter in the world by many.
Pacquiao landed double the powerpunches and twice as many punches overall on the 2nd round, how can JMM have won 11 rounds?
it doesn't glorify Pacquiao to say that JMM fought better in the 2nd fight that the first. Whats the difference? Whats so glorifying in that? the haters are getting paranoid
Best performance for me means a perfect fight, and with that in mind, I would say JMM's peak best performance was against Medina. I would not say his two fights with Pac since there were moments that he was outclassed there. Remember he was KD'ed numerous times. I am inclined to put his fight with MAB but again, it was a close fight until the second half, where he really take off (and even that he was KD'ed) and zoomed for a close but decisive victory. In the Medina fight, it was a showcase of excellent offense and defense, and ended in an impressive KO, against a solid champion.
Both fights vs. Pac and MAB. He showed he was in their class in all of these fights. But he looked like a stone killer against Jimrex Jaca.