Favorite Guest Star would have to be Phil Hartman. My favorite episode is either A Streetcar named Marge or Last exit to Springfield. Actually, pretty much any episode in seasons 1-8.

My favorite ep would have tobe when Krusty was bi-polar and Mr. Burns cancelled the health insurance. If anyone knows which ep this was, I would love to buy it.

My favorite guest stars are Kelsey Grammer and Phil Hartman (shocker!), but you also gotta love Fat Tony (Joe Mantegna).

My favorite episode is Missionary: Impossible where Homer becomes a missionary in the South Pacific to hide from PBS. Homer getting chased down the street by the PBS characters is hilarious and cracks me up every time.

"Elmo knows where you live!"

"But I don't even believe in Jeebus!"

"I'm not NOT licking toads."

I agree with the obvious choices of Phil Hartman, Kelsey Grammar and Albert Brooks but I can't believe nobody mentioned James Woods in "Much Apu About Nothing", They used him brilliantly as a more extreme version of himself. It was HI-larious.

Brian "Patrick Stewart (Number One from Stonecutters)". Good call. I love that ep. I still occasionally mutter, "we doooo, we doooo..." and nobody knows what I'm mean.

"Hi, Billy Corgan, Smashing Pumpkins"
"Hi Homer Simpson, smiling politely."

The whole hollapoluza (sp?) episode was gold, but that's my favorite.

Favorite recurring guest star: Kelsey Grammer and Phil Hartman (love that Troy McClure)

Favorite single appearance guest star: Patrick Stewart (Number One from Stonecutters) and John Waters (John from Homer's Phobia)

Favorite Musical Guest: Red Hot Chili Peppers (we want chilly willy!!!)

Favorite Episode: I don't have one but I have a range, like anything from the early 90's to mid-late 90's were the funniest episodes I remember, but the "Duffless" episode where Homer gets a DUI and loses his license. He gets kicked out of AA because he confessed that he was so desperate for beer he would eat the dirt underneath the bleachers.

One of my favorite quotes: (it's out of my favorite ep range, but it's still funny)
Homer from The Joy of Sect: Wait, I'm confused about the movie. So the cops knew that internal affairs were setting them up?
Glen: What are you talking about? There is nothing like that in there!
Homer: Oh, you see when I get bored I make up my own movie. I have a very short attention span.

My favorite guest stars would have to be All 3 Beatles, George from B-Sharps, Paul from Lisa is a Vegetarian, and Ringo from the Marge Artist episode.
Michael Jackson as the crazy man in Stark Raving Dad
Dustin Hoffman as Lisa's Substitute,
Conan O'Brien as himself in
Chicago's own Joe Mantegna as Fat Tony... in every episode he shows up in.
Albert Brooks as Hank Scorpio
U2 as themselves in Trash of the Titans
AND FINALLY... THE GREATEST SIMPSONS GUEST STAR
Phil Hartman as Troy Mclure in the various episodes(I know he's voiced countless others but Troy is my favorite)

As far as my favorite episodes go, some classic ones include Oh Brother Where Art Though, Brother can you Spare 2 Dimes, There's no Disgrace like Home, Who Shot Mr. Burns Parts 1 and 2, Homer and Apu, Eight Misbehavin, City of New York VS. Homer, The Two Nahaseepetilons(spelling?), Bart's Comet, Much Apu About Nothing, Das Bus, 30 Minutes Over Tokyo, Lisa the Skeptic, Homer Defined and finally Burns Baby Burns.
Of course this is just a small list, there are countless other episodes to include.

I agree with the Kelsey Grammer love. I loved Leonard Nimoy in Marge Vs. the Monorail. Ricky Gervais also did a good job as a recent guest star. Glenn Close as Homer's mother. The incomparable Phil Hartmann. Donald Sutherland as the museum curator in Lisa the Iconoclast.

My favorite episode, laughs-wise, is when Homer purposely gains weight to go on disability. "I don't want to look like a freak. I'll just take the muu-muu." "Why did I leave you in charge?!?"

Maybe I'm just being sentimental, but my fave guest is an early one: Liz Taylor, who voiced Maggie's first word. I love the episode in which Bart sells his soul to Millhouse. That ep also had Moe opening the restaurant that deep fried everything. Great stuff.