I would actually say the first Tarver fight. Here's a guy coming off a historical fight and has to drop 20+ pounds of muscle just to make weight. He's fighting against a self proclaimed rival (Tarver) who has spent the last 20+ YEARS dreaming of this moment, and a one punch southpaw at that. During the fight, it is obvious that physically, Roy is not the same. And yet, like all the other ATGs, RJJ STILL dug down and found the energy to rally in the last 2 rounds to secure the victory. It's the fight that (at the time) answered the one remaining question of his career: did RJJ have the 'heart' to come back and will a fight purely on 'will'. After that performance, combined with the Ruiz fight, I don't think there was any argument of whether RJJ belong in the conversation as a top 5 P4P. I simply ignore everything that happened since.
Best performance; Clinton Woods Best opponent; James Toney Greatest achievement; Winning the Heavyweight title off John Ruiz.