Hard to decide, but if I have to pick one it’s probably the Homer evolution intro. Especially Moe going backwards cracks me up every time!
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The "Circus Line" couch gag for sure.

Apparently it's the most used gag during the standard definition era, which is maybe why it sticks with me, but it's also the final gag after the couch gag medley in the 138th episode spectacular, one of my favorites.
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I always liked the one where the family was already sitting down when everyone came in, and they just kind of awkwardly looked at each other.
Easy. The Don Hertzfeldt one. It's not even a close race.
https://m.youtube.com/watch?v=m78gYyTrG7Y&pp=ygUXZG9uIGhlcnR6ZmVsZHQgc2ltcHNvbnM%3D
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