Diverse — This includes things like characters of different cultural, religious, and socioeconomic backgrounds, LGBTQ+ characters, and neurodiverse characters, and so on. I prefer books that are less about a bisexual or Mexican American or whatever character and more about a magical world or an epic adventure that happens to feature a bisexual or Mexican American character. Obviously, it’s great to have the former, but I just like to read about characters (who might be like myself) who get to go on adventures and whatever, and whose differences don’t define them. 

Relatable — This kind of goes hand in hand with the diversity. I like to see characters like myself in the book. Of course, I don’t have to be exactly like the main character but it does make the read much more enjoyable if I can relate to at least one aspect of them.

Believable — Not necessarily realistic. Even in a fantasy world, the book needs to have a clear magic system and the problems need to be solved in a way that makes sense — like the MCs can’t just suddenly defeat the villain through the power of friendship or whatever, unless that actually makes sense with the plot and the established magic system. And there can’t be any major plot holes.

Original — I don’t enjoy seeing the same tropes and character types over and over again. I like books that offer a fresh perspective, and feel like the author is really passionate about their story.

Beautiful Writing — Like Poinsettia said, I like books where the writing is more than just effective. Books with gorgeous writing and imagery that just puts you in the scene alongside the characters and makes you want to reread it just to soak up all the beautiful writing.

I'll be back if I think of anything else! 

Complety agreeing with everyone who posted after me, with Celine

(you guys just put it into better words then I would have) 

--very complicated and realistic characters (usually meaning they're very imperfect). Six of Crows and The Raven Cycle do this best, but Harry Potter is good too.

--on the topic of characters, I love well-written friendships in books. Those same three series have some of my favorite fictional friendships. The Perks of Being a Wallflower is really good too, and, of course, the fox and the little prince.

--ensemble casts! 

--I do like a spot of romance in my books. Especially if it starts as a friendship. 

--boarding schools, for some reason

--urban fantasy! i think that's what it's called? when the magic is in the everyday world. Again, The Raven Cycle. Basically everything by Neil Gaiman. I enjoy some high fantasy, but I read less of it.

--a bit of humor is always appreciated. straight-up comedies don't tend to be my favorite books, though. I like them, but a book has to make me think as well as laugh for it to be a favorite.

--like everyone says, beautiful writing. lots of figurative language. Markus Zusak and Maggie Stiefvater are good at this. I Capture the Castle, too.

1: good, funny dialogue 2: multiple main characters that are all well-written and diverse 3: any kind of fantasy! (Six of Crows, Skulduggery Pleasant, Ranger's Apprentice, Lockwood & Co, King of Scars)