I think the best ever putdown was from Homer the Heretic..
Rev Lovejoy ".....protestant, catholic or....miscellaneous" Apu "Hindu! There ARE over 200 million of us!! Rev Lovejoy "That's super"
Also quite a classic line - it was in the bible stories episode
Bart: "Ralph! I thought you were dead!" Ralph "Nope"
acompanied by Ralph's trademark blank look, so blank it borders on insanity.
What are your favourites?