My favorite chapters are based on either:
1. They have a quote or moments that *I* consider pivotal or something that makes me think deeper
2. There is some sort of character growth for character/s that *I* like
3. There are some kick ass dialogues or moments where the bad guy, so to speak, GETS IT.
I havn't read COS (I just started it recently again), POA, GOF and even Phoenix in quite a while so chances are I'll miss things that I liked from them...I'm only gonna do this for book 1, 2 and 3 right now or else...trust me, it's gonna get LONGGG since you've also asked us the reasons for liking them.
Book 1: The Boy who lived (I love the whole explanation of how the Dursleys feared 'abnormality' and wanted everything 'normal', The Journey from Platform 9 and 3/4 (he meets Ron/Her, obv. that's a great moment...Harry finds his first real friend), The Mirror of Erised (Great quotes).
Book 2: The Burrow (really lovely how Ron, despite his insecurities about his poverty opens himself and his house to Harry and gives him a much desired family, Harry did get lucky), Gilderoy Lockhart (Let's just say, Lockhart is entertaining in more ways than one), Mudbloods and Murmurs (Ron's 'eat slugs' moment with Malfoy, we get to know somewhat that the wizarding world isn't perfect, there's bigotry here too, human nature is the same everywhere - one area where all magic is powerless), The Polyjuice Potion (the most fun chapter)...that's it really, not many that I loved.
Book 3: The dementor (simply the idea of a 'dementor' is brilliant), Talons and Tea leaves (I do loveee Divination and Prof Trelawney), The Boggart in the Wardrobe, The Patronus, Snape's Grudge (just cuz the story is moving forward lol), Cat, Rat and Dog (I honestly have NO idea if this is the chapter where R/H resolve their whole dumb conflict about Scabbers and Crookshanks...but phew was glad when it was done), I do like the chaoter where they rescue Sirius but honestly, I've not been a fan of any of the Marauders that much except for Lupin...so it was interesting as far as the story goes, but this books isn't really one of my favs.
See, this got long even with the 1st three books that I don't like much...4-7 are my favs so it's gonna get longer...will be back shortly.