For me personally it’s “the beetle at bay” purely because of how much second embarrassment I get when reading about Harry and Cho’s date. For me it is the not the worst chapter but the most uncomfortable one which makes hate reading it.
Deathly Hallows, chapter 33, The Prince's Tale. It's my favorite chapter from the entire HP series, and definitly the saddest, in my opinion, as Snape is my favorite character