I mean obviously my easiest go-to, it was episode of the decade the day it came out and wasn't surpassed since. I'd say the entire madoka franchise strays from episode 10.
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I agree with all of these but still think some of them are more carried by one scene and I don't feel like they have all a huge lasting impact but pingpong, K-ON or haruhi.

Steins;gate episode13
Made in abyss episode1
clannad after story episode18
TTGL episode15
Haibane renmei ep8
These come to my mind first as 10/10 eps.