I would say my best RP wasn't a single moment, but an entire huge-ass RP I had with a friend of mine from high school. We knew each other irl, so that made planning things pretty easy, and we RPed on AIM, so it moved pretty quickly as well. The RP spanned numerous story arcs and had 3 generations of characters in it. There were many great villains, many greater heroes, and many tearful goodbyes. Plus, at one point, we stuffed a portable nuclear device into a necromantic dragon's mouth and blew him to kingdom come. These were good times...