For years I had an answer to the question of what my favorite Harry Potter book was. After Deathly Hallows came out, it was my answer, but I usually specified that Prisoner of Azkaban was a close second. These days, I don't really answer the question because it's not as clear cut for me as it was in the past. Or maybe I just don't see choosing a favorite as important anymore. That sort of thing seems to apply to more things for me now than in the past. I really don't single out things as my number one favorite anymore. I'm far more likely to lump a lot of "favorites" together, which some people may see as diluting the term "favorite." I don't really think of it that way.
I'm not sure when I stopped having a clear cut favorite book. I didn't notice the shift just like I didn't notice when exactly I stopped thinking of Hermione as my number one favorite character and whatever else. Over time the hierarchy of Harry Potter books that I had in my head just faded until I honestly can't remember what order exactly I used to list them off in. I have a vague idea.
For instance, I know that Goblet of Fire was always last. It was right from the first time I read it, but now I feel like it's been long enough that I should give it another chance. It's the one I'm currently reading for my Harry Potter re-read. I'm only a few chapters in, but I'm going to see if it manages to change my point-of-view this time around.
Of course, I still have a soft spot for endings, which I've mentioned countless times before just about everywhere on the Internet. The final books or the final episodes and whatnot tend to be my favorites. I enjoy them immensely at the very least, unless I can't stand them (which is rare). For that reason alone, I know Deathly Hallows will always be special in my mind in a way the other Harry Potter books can't achieve, but I don't feel it necessary to label it as my favorite Harry Potter book anymore.
This does present a bit of a conundrum if someone asks for my favorite book and won't let me just say "the Harry Potter series." In that case, I'd probably just keep dropping Deathly Hallows as my answer, but that's largely because it's an easy answer. And because I like satisfying endings. There's that too.