I haven't played too many RPGs based on an intellectual property (IP). There are quite a lot out there now. In the early days I remember we were constantly wishing that someone would do a game based on X. But for this challenge question I'll have to say Call of Cthulhu. Star Wars was a close second, but I haven't played it a lot.
I first came across the Call of Cthulhu novels around the end of my college days. I was immediately thrilled with them and quickly bought up every one I could find. Naturally when an RPG appeared for it I pounced on it. I'm in a CoC game now, after all these years, and it's still fantastic.