I'm going to say "Sand Castles in the Sand", which a lot of people claim is one of the best episodes of the post-movie era. While it's a strong episode and creative, for me, it's not even the best episode of Season 6. That honor would go to "Suction Cup Symphony", with "Spongicus" close behind. Both of those are fantastic episodes.
My Leg! is one that comes to mind. It’s just a lot of screaming ‘My Leg!’ that has decent moments in it. Even though I don’t like Season 11 that much, there are episodes better than it (ChefBob, Squid Noir, etc)