As one of the longest running TV shows of all time, The Simpsons holds many notable distinctions -- not the least of which is the sheer number of guest stars who have appeared on the program during the course of its 25-year history. Literally hundreds of celebrities (Wikipedia credits 652, including those in The Simpsons Movie) have been committed to cartoon form, either playing one-off characters or voicing versions of themselves. In the best cases, a confluence of great writing and impassioned voice acting creates something highly quotable and truly memorable. Here are the best of the best -- ten guest stars whose brief appearances have become iconic over the years. With only their voices to represent them, they delivered performances that stood side by side with those of the regular Simpsons cast -- and really, what higher praise is there than that? Just a note -- this article focuses exclusively on guest stars who played fictional characters (maybe another list with stars voicing themselves will follow). Also, note the omission of the legendary work of Phil Hartman and Marcia Wallace -- their numerous appearances on the show make them regulars more than guest stars.