Its kind of difficult to seperate rules from background. My leaning is towards simple and intuitive, but it depends upon how you play. As a teenager, I played so much MERP I had literally memorised all the tables and could run a game without the book!
Dragonlance was the first game I played so thats always a favourite, but I love Paranoia, a game where you were literally encouraged to throw all the rules out the window, especially the ones about physics and thermodynamics and the like.