Dunking on the later Potter films isn’t unpopular at all. It’s widely accepted that the series peaked with 3.
However, though 3 is definitely a favorite of mine, I consider The Half Blood Prince to be just as good for completely different reasons. This is the rare time where I genuinely enjoy the dark, murky, and at times sickeningly green/brown color palette because it is appropriate for the darkness of the story being told. The Half Blood Prince isn’t supposed to be pleasant. However, there is a stark filigree beauty in the almost monochrome depiction of the Hogwarts castle and the exquisite details of the crystal cave. It evokes repulsion and enchantment in equal measure, and it is alone among Potter films in this regard.