I don't think that I have a favourite chapter, I don't even read the chapter titles, I just skip them (I don't know why, I've always been doing that - whether in literature books or in study books... I used to have problems then when I learnt everything I should except the titles, it confused me sometimes...). But I have some favourite scenes, though I have no idea in which chapters they are.
That chapter when Snape loses the Order of Merlin is funny, I love seeing him losing his coolness.
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As for other chapters, my most favourite ones are usually those that have Snape in it (e.g. Snape duelling Lockheart, very funnily described) or Sirius, but also some other characters that I like.