I rarely, if ever, respond to comments about my trivia questions, but I thought it was unfair not to explain why Albert Brooks is the right answer.
Phil Hartman is not a "guest" because he often would play characters who were ancillary to the plot of the episode (Lionel Hutz, Troy McClure). I don't include Jon Lovitz, Marcia Wallace or Joe Mantegna for the same reason. That only leaves Kelsey Grammar with more appearances than Brooks. In this case, Grammar played the same character changing his "guest" status to a recurring role the more he appeared. I don't apologize for my reasoning but I do apologize that my question was not more clear. I will try to rectify that mistake in the future.

Brooks didn't appear as himself either. He played 4 (I think) different characters over 5 episodes.

Grammer appeared as a character not a guest ("himself").

I dunno about raising the bar for animated films in general. Perhaps for those based on TV-shows, but the animation can't compete with the latest pixar movie, or shrek.

Though I have a bias towards 3d animation, a lot of the more robust scenes in the simpsons movie were CG if I'm not mistaken they call it "toon shading".

I guess that depends on how you define 'celebrity'. Marcia Wallace and Joe Mantegna may not be 'celebrity' enough to count, but I would have said Kelsey Grammer was a celebrity and he's appeared 9 times to Brooks' 5, and as the same character each time, no less.

How is it Albert Brooks exactly? He's only been in 5 episodes. He may be the only guest to return without playing the same character, but Jon Lovitz has been in 9 episodes playing 8 different characters.

Really? Albert Brooks has been on more than Marcia Wallace and Joe Mantegna?

Tress McNeille or Pamela Hayden?

Yeah, I'm guessing Marcia Wallace. By the way, why IS she always credited as a 'guest'? Is it just some running gag, or is there some contractual reason?

Marica Wallace