I know it might be unpopular to say, but I remember Mel Gibson had a guest appearance. And it was golden.
I agree with most of them, except for Elon Musk. That was a terrible episode, although not quite as bad as the Lady Gaga one (*shudder*). I concur with Saltyk that the Mel Gibson guest appearance was far better.
And of course, in the Michael Jackson episode, while it was him doing the voice, he had an impersonator along to sing the songs.
I think special mention has to be made to Sir Stephen Hawking
Elon Musk was actually kind of awful. Delievered his lines terribly.
Glenn Glose was robbed.