For me it's sanji baking a cake for Big Mom.
Oda unironically thought this was peak fiction:
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Honestly I would say the timeskip itself,
Fishmid Island especially the reprehensible storyline with Sanji,
Usopp vs Sugar,
Sanjis writing on Punk Hazard,
Zoro never getting Nidai and Ame No Habakiri being pointless,
Enma stealing haki and seemingly being sentient with no explanation but "curses swords are a complete" apparently,
What happened to Law and Kidd post Wano, the wastage of Hawkins, no Brook vs Apoo etc.
Nami's design post Tit Surgery and Robins and thus their Egghead outfits too especially. Ironically Stussy had the same outfit but got censored in the anime.
The creepy shit with S-Snake and Bonney
Vegapunk in general
Many important battles being offscreened and we still barely know anything about them
No Logia awakenings nor Logia clones fruits
The other 3 Seraphim being entirely pointless so far
Green Bull offscreen capturing Weevil and Weevil not getting a chance to even be a character
Odas completely apathetic, shameless writing of Usopp and even lack of writing for him
The RHP being ao cowardly, cruel and hypocritical
Garp vs Kuzan was underwhelming
BBP going to Egghead was almost pointless and we still don't know why they really headed there, there's no way they knew beforehand that Saturn was gonna be there? Only Augur and Devon and Augur was just a teleporting plot device as he was in Law vs BB too.
Some of the revealed DFs for the BBP and Lafitte being barely a character and doing barely anything still and also still no DF reveal.
Don't even get me started on the shittiness of Burgess and San Juan Wolf or the pure incel tier writing of Oda saying turning into women is an actual disease.