My favourite is the Deathly Hallows shortly followed by the Order of the Phoenix. Although I liked the first three books, I feel the writing style is a bit too young. Not for who its aimed - that's perfect - but for an adult reader. I found the sixth book and bit annoying, and while I liked the fourth, the seventh was the final holds all that tense excitement.
The only spin off books I've read is the Tales of Beadle the Bard.
I very much prefer the first one. I felt that after the stratospheric success of that, her editor stopped editing and didn't do his/her job properly. Particularly as the books got bigger and bigger, a red line drawn through all the surplus paragraphs would have been a good idea!
I find this a lot with really successful authors. Their first novel is wonderful then subsequent ones are sloppily edited and much harder to read as a consequence. Proper editing would have resulted in much tighter novels, with really gripping action and better suspense, in the Harry Potter series.