18. Paul and Lisa McCartney (S7 E5 - "Lisa The Vegetarian")
Paul and his former wife Lisa weren't much of a surprise in that Paul, George and Ringo of the Beatles had all already made an appearance by season 7. Matt Groening was a huge fan of the Beatles and he was able to fulfill 75% of his dream of meeting them. The couple's appearance was of a serious note as they become spiritual mentors of a sort for Lisa, who makes a committed decision to become a vegetarian. The power of Sir Paul really manifested itself here when it is discovered that the McCartneys condition for appearing was that Lisa would remain a vegetarian for the rest of the series. I don't know about you but when you have an aging musician deciding your cartoon diet for the next 17 years and counting...well, that's not too cool, Sir Paul, not cool at all.