"What RPG based on an IP did you enjoy most? Give details."
To be honest, I'm not the biggest fan of the Cortex system. It feels a lot like a more complicated take on Savage World's dice-increase-by-size mechanics, while tacking on a significant series of ancillary rules that require an encyclopedic knowledge of the ruleset to run anything smoothly.
That said, the best time I've ever had running an existing IP game was with Serenity RPG, and it's all because of the fanbase.
Joss Whedon's Firefly/Serenity space cowboy universe is, well, almost universally hailed as one of the great original concepts for television in the last decade, and memes lamenting its cancellation still propagate my facebook feed to this day. These are dedicated people, full of fervor and excitement for the setting, and more than willing to do the legwork to feel right at home there. By luck, or pure social statistics, I have a goodly amount of friends who are a few wardrobe pieces away from being browncoats themselves, and they were more than willing to jump into a quickly written adventure that I wrote to take place about a year after the events of Serenity.
I think it's a testament to the 'verse's original look and feel that each player knew how their character would fit into the setting, and it's really rare in most games to know that everyone has a cohesive view of how everything works. Not having to run through an introduction to the basic universe details, or stop every few minutes when a new piece of terminology comes up, even being able to explain new concepts by referring to knowledge players already had by experiencing the show...well, if that's not a windfall for a GM I really don't know what is.
It's one of my regrets as a GM that my Serenity RPG campaign only lasted three or four sessions; the players really enjoyed it, I enjoyed writing for it. Perhaps I can return to it sometime in the future, but then again, there are a lot of things I need to return to in the future, and Diablo III isn't helping.