So which guest voice actor do you think is best?

This can be a guest actor as themselves, such as REM.

As a character, like Lisa Kudrow.

As a recurring character, like Kelsey Grammer.

Appearing as many different characters, like Albert Brooks.

I think that Patrick Stewart as Number One, Kelsey Grammer as Sideshow Bob, Phil Hartman as Troy McClure and Lionel Hutz and Jackie Mason as Rabb Krustofski.

What about you?

I know it was only one line in aHalloween episode, but my fave was Matthew Perry!

I would say REM

Patrick Stewart as Number One was my personal favorite.
And Kelsey Grammer as Sidshow Bob is always gold.

Two others that always stick in my head are James Woods and Mark Hamill.
BRG

guy who does the voice for Fat Tony, he's gold!...

Phil Hartman was great, its such a shame he died ...

REALLY?! i didn't no that! i can't remember whose voices he did though

Yep, he was murdered by his wife, who killed herself 2 hours later.

Anyway he did Lionel Hutz and Troy McClure. Two of the best characters in the whole of the shows history.

As well as other one off characters such as Lyle Lanley, Evan Conover and Tom.

Kelsey Grammar as Sideshow Bob cannot be beaten
Although his brother Cecil comes a close second.