hbp... no question.
i felt like ootp took a really long time to get nowhere, a giant set up for the glorious read that is hbp. of course, harry's general attitude did get a little old, and definitely got tired of hearing the whining. i also didn't like the distance between harry and dd in ootp, hence one of the main reasons i loved hbp.
also, i just found hbp to be hysterically funny (not all the way through of course.) i just had a great time reading it. it is a great book that can make you laugh and cry all on the same page.