I just watched the episode where Burns sends Homer back to college the other day, as I’ve been going through my new DVDs. There’s a hilarious throwaway bit in there I’d totally forgotten about. After Burns tells Homer he has to go back to college:
Burns: “Also, you must find the jade monkey before the next full moon!”
Smithers: “Uh, actually, sir, we found the jade monkey. It was in the glove compartment.”
Burns: “And the driving maps and gloves?”
Smithers: “Also in the glove compartment.”
Burns: “Excellent. It’s all coming together!”
They walk away.
I’m paraphrasing there, so if you must nitpick an occasional incorrect word, please know I don’t care. 
Anyway, for “best episode,” I have to go with Last Exit to Springfield, which I also watched recently (DVD again). I remember turning it off thinking, “Wow, that was the best Simpsons episode they ever did.” Of course, now I can’t remember specifically why I thought that - thanks to TV, my attention span is so short, I can’t remember what happened seven minutes ago! I do believe that was the episode with the famous “I tied an onion to my belt, which was the style at the time” speech, which went on for so long that it stopped being funny, then became really funny again. Even Burns was bored with it.
The one with Lisa getting braces is also right up there, thanks in part to the insane dentist. Hilarious.