Order of The Phoenix is a really good fantasy book, I don't see the argument against it.
Ultimately I think the biggest problem with Harry Potter, and many fantasy books in general, is the general perfection of the protagonist. Think back: Harry is almost never outright wrong about anything. Nearly every book follows the same arc: Harry learns some secret, tries to tell everyone, and is lambasted or punished. He then breaks a variety of rules that he deems contrary to whatever truth he has discovered, and is lightly punished. And finally, after vanquishing whatever evil he warned everyone about, he is rewarded by the doubted who then proceed to apologize to him.
OOTP initially seems like the same trope is being deployed, but by the end Harry is not only wrong for once, he pays for it dearly (Sirius' death).