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.
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.