Honestly, I don't think there’s one single 'official' fan ranking that everyone agrees on—every poll and community seems to have its own flavor. You see 'Prisoner of Azkaban' topping lists on Reddit and Tumblr constantly, and I get it; the Marauders' backstory and that shift into darker territory is a real turning point. But then you go to some of the older forums, or talk to people who were adults when the books came out, and 'Goblet of Fire' gets a lot of love for its scale and that stunning maze sequence. The rankings are less about a decree and more about spotting patterns in the noise.
For what it’s worth, most aggregated fan polls I’ve lurked through tend to settle into a loose tier. 'Azkaban' and 'Half-Blood Prince' are usually in the top tier—the latter for the depth of the pensieve memories and the tragedy of Dumbledore. Then you’ve got 'Goblet' and 'Deathly Hallows' in a strong middle, with 'Hallows' sometimes polarizing because of the camping chapters. 'Sorcerer's Stone' and 'Chamber' are often lower, but I see them get a nostalgia defense squad, especially from readers who started there. 'Order of the Phoenix' is the real wildcard; some rank it last because Harry’s angsty, others put it high for the DA and the Department of Mysteries fight. So the 'official' ranking is basically a consensus illusion—it changes depending on which fans you ask and when you caught them.