Jd Smith1
Hero
For me it's Star Wars.
I only played WEG's Star Wars RPG, a couple of times, when it first came out, but thought the game was really cool. The setting is fantastic, and like many others, I am a big fan of the Star Wars movies (all of them).
But I don't read extended universe novels, etc. and have always been concerned about my ability to run a Star Wars game, where it's quite likely the other players will know the universe better than me. Then you run into issues associated with running games based on well-known intellectual properties (e.g., Middle Earth), and it's just too much.
So Star Wars remains my Moby Dick, and likely will always be so.
I looked into SW recently, and decided against running it for that exact same reason.