I've officially reread all three books at this point, and it's let me reflect a little on the series. It's clear to me on this reread that Fire is my favourite book in the series, although Graceling is only a hair behind at second, and Bitterblue a hair further behind in third. What it really comes down to is this: Fire makes me cry the most.
I know, I know it sounds really hypocritical of me since I talk so much about how I procrastinate hardcore on sad books, but there you are. And I would like to add there's also a big difference between sad books and books where sad things happen. In fact, it's a similar distinction I make in general between YA and adult literature - YA literature is almost always hopeful, even at it's most tragic. Most adult literature I come across is sad in that ever pervasive, hopeless, drudgery way. If you're going to make me sad, you damn well better get a bucket of tears and heart palpitations from me. Just saying.
THERE ARE SERIOUS SPOILERS AHEAD. I would recommend scrolling really fast all the way to the bottom of your post and cautiously making your way up until you see the end spoilers words if you want to read the bits of the post without them! In the interest of not spoiling the really really spoilery bits (and not just my reactions because I don't think you can make images sort of invisible) I will make some of what I write under the reactions only visible if you highlight it.
My progession on reading Fire:
1. First half, oh yeah I remember how much I love this!
2.* About halfway through...oh god. Oh wait. Doesn't Archer die?! WHY DID I REMEMBER THIS JUST NOW??
6. Cry because the book ended.
I would also like to note, that while I am an absolute crybaby in most books, I very rarely cry because a book has ended. If it's a decent book, I might put it down with a sigh of satisfaction and move on. If it's quite good, I will probably lie in bed and think about it for a while. If it's really good (a la Graceling and Bitterblue), I will become despondent for a good hour, think about all the characters and the worldbuilding, and whyyyyy did it have to end?! But it's rare that ending the book actually makes me cry. Off the top of my head, I can only think of the last Harry Potter book that put me there.
I'd make the educated guess that Graceling is the general favourite of the series. My guess is that Bitterblue is less popular because it is significantly less action packed when compared to Graceling and Fire, and at 563 pages, it's also a good 100 pages longer than the other books. I still loved Bitterblue, and I love, love, LOVED, that it dealt with the aftermath of Leck. My respect for books that deal with things not magically being ok afterwards (Different book spoilers: Mockingjay, Quintana of the Charyns) is insurmountable, and I think more of them need to be out there.
My guesses for why Fire is less popular are very different. For one, I think people probably expected more Katsa and Po (and who wouldn't want more Katsa and Po?) and it was probably a bit of a shock to find yourself with completely different characters in a completely different land. It's also very outspoken about rape in a way that I think can both empower and infuriate women. And from here on out it's probably going to be a little spoilery, at least about the world building. It's always tricky to talk about rape, especially when it is built so deeply into the world building. It's always a fine line to walk, and inevitably you are going to upset people. I'd argue that as a whole, (and there are individual times when I might not agree, and I'm still uncertain how I feel about an entire book that deals with violence towards our heroine) that Kristin Cashore does this in a respectful way that doesn't use rape simply as a plot device, and that also opens up a dialogue about what makes people good or evil (or neither). While at times it may seem like the book is saying it's Fire's fault that the men are acting this way (and that's never an ok thing to believe in my book), I'd also like to point out that usually, any time this was mentioned - the men who look to act this way against Fire generally were this way beforehand, it just happened more suddenly and violently. At some point in the book she talks to Brigan about how she turns most men away from her tent, but the men that are truly violent, she let's attack her so that Brigan will dismiss them. This is still a mixed bag, and I totally understand why a lot of people were really upset by the way this was approached. I tend to take the positive out of what was said, but I'll get to that later.
(Sidenote: One of my only quibbles on this point is that it's mentioned multiple times that it's worse for Fire because she's a woman, with the implication only women are desired. It's not only men who feel desire - I'm sure there were women who lusted after Cansrel. Yes, physically speaking, it's generally much, much less threatening, but there are also gay men. My point is that worldbuilding wise, it doesn't make sense to me that only women are desired. It does make sense to me that it's much scarier for a woman - specifically a woman like Fire vs. a man like Cansrel. But it's weird to me that it's pointed out on more than one occasion that Fire is desired because she's a woman, and that these problems don't apply to Cansrel)
I'd also like to say that Fire almost never blames herself for the way men act towards her. Archer and Nash are perfect examples of this. Both of them act in a manner that is wholly unwelcome to her, and she let's them know that it's up to them to get themselves under control. I also think that Archer and Nash are brilliant studies in what makes a person good or evil. We seem examples, like Leck and Cansrel, of people who are pure evil. This is juxtaposed against people like Fire who is a "monster", but who isn't inherently evil. Then you've got Archer and Nash who seem like mostly decent men, but are more or less unbearable around Fire. (I have really complicated feelings on Archer. I kind of hate him as a person, but he's not all bad. Spoilers: And I was still extremely upset when he died. I don't really get it, to be honest. I think he really tried to be good, but....I don't know. Complicated feelings.)
All of these are reasons why I love Fire (and why if you haven't read this series, GO READ IT NOW. I know plenty of non-fantasy lovers who loved it if you aren't convinced). I never really got to why this is my favourite book of the series - I talk about why I love it, but none of this is what makes this book so personal for me. This post has gotten so insanely long that I will have to wait for another post to get to that though :)