probably OOTP what with fred and george calling Umbridge an old bat, the anarchy that goes on through out the castle when Umbridge becomes headmistress, McGonagall lending Peeves her walking stick to hit umbridge and telling him which way to unscrew a chandelair, flitwick slamming the door in umbridge's face, McGonagall and Umbridge in the careers advice, also Luna Lovegood's blatant honesty, the weasley wizard wheezes items, and arthur and Molly at st mungos when he tries to explain he had muggle stitches, and the regurgitating toilets he had to sort out at the beginning, so all in all OOTP is the funniest book I think.