I can never actually name a #1 best all-time episode. “Duffless” and “Brother From Another Planet” are top eps for me. “You Only Move Twice” is kind of the last truly brilliant episode, unless somebody can prove me wrong.
Can’t prove it, and this episode is better, but both The Cartridge Family and Bart Star are great Season 9 episodes. I’d say they were the last.
JEDIT: City of New York episode is pretty good but I like these two better.