Hoo boy, there are plenty of candidates.
How about in Understanding Willow with the reason behind Willow and Amity's friendship ending being revealed, and Amity's explanation and new resolve to not let Boscha and her crew pick on Willow anymore?
Or the climactic witches' duel and aftermath in Agony of a Witch, where Eda sacrifices the last of her magic to save Luz and succumbs to the curse, seemingly once and for all?
Or in Young Blood, Old Souls, where Luz and Eda express their love for each other and Luz believes she's watching Eda being raised up to her death?
Or in Separate Tides, where Luz expresses her guilt and belief that she's a burden, and Eda thoroughly debunking that?
Or in Echoes of the Past, where King finally realizes the fiction of his supposed backstory and subsequent breakdown? (We'll call this one the runner-up. Alex Hirsch's finest performance, I swear.)
Or in Keeping Up A-fear-ances, where Gwen realizes the harm she's caused both of her daughters and her newfound resolve to do right by them?
Or in Through the Looking Glass Ruins, where Luz and Amity tell each other they do stupid things around each other? (Shortly followed by the kiss!)
Or at the end of Luz's bit in Hunting Palismen, where Eda drops the Palistrom wood in Luz's lap and assures her they can start carving when she's ready?
Or in Eda's Requiem, with the performance of the titular requiem, Raine assuring Eda her kids still need her more than she realizes, "You know I can't stand an audience," and King surprising Eda with the name change? (Wow, there was a lot in this one!)
Or in Knock, Knock, Knockin' On Hooty's Door, with:
- King's fear of never finding out what he is and anger at his dad for leaving him?
- Just...the entirety of Eda's dream sequence, culminating in finally making peace with the Owl Beast?
- A heartbroken Amity believing she was just rejected by Luz in the most painful way possible, and Luz horrified to realize she dun goofed?
(Sorry, I was very tempted to gush about the entirety of K³OHD again)
All good, emotional moments worthy of mention. For my money, though? It's gotta be near the end of Yesterday's Lie with the exchange between Luz and Camila in the rain.
You can feel Camila's fear about Luz being in a strange world where she can't be reached. ("Is this the only way I can touch you?" still gets me) You can also tell how hurt she feels when she believes she pushed Luz away to the point of escaping to this strange world, while it's also clear that Luz never meant it that way, and in fact Camila is the main reason she's trying to get back in the first place.
I imagine many people will cite this episode and scene for this particular day, and with good reason. This scene still haunts me in the best possible way, and I very much want them to get to a point where they can start healing.
Oh, another honorable mention from the same episode is Vee detailing her troubled past and Camila accepting her as she is.
Whew! That was another long one! I like it when I have a lot to say. Hopefully other people do, too.