Even if my favourite remains Prisoner of Azkaban, which I think will be hard to overcome, I have to say that maybe Order of Phoenix is the best-written, I mean, so full of descriptions (The Ministry, The Hearing, The DoM, Girmmauld Place, ...), of details, of really great inventions, such as the portraits walking through their pictures and so on... Really great...
Yes, I think it's the best prose but not the best plot, even if at a great level, almost at Goblet of Fire.