For me, I think it would have to be either:
The Mists of Avalon by Marion Zimmer Bradley, which I read at my grandmother's house the summer I was 11. It completely blew my preteen mind...despite the fact that it's over 800 pages, I read it a couple of times a year from then on. I also dabbled in Wicca and became an avowed Anglophile throughout middle school, basically because of the book (I know!). I still love Arthurian legends and stone circles, and I think the book definitely contributed to my feminist consciousness, as it's all priestesses and Goddess power and fuck patriarchal religion. I haven't read it in maybe eighteen months, but I'm due.
The Ordinary Princess, a book I stole from my third grade classroom and read all the time throughout my childhood. It's by M.M. Kaye, who wrote The Far Pavilions, and it's probably the most charming fairy tale ever. It's a book about a princess named Amethyst who is "cursed" at her christening with ordinariness, which means she is plain and plucky and smart. She's nicknamed Amy (so normal!) and she doesn't grow long blonde hair or pale white skin like her perfect princess sisters. When her parents try to marry her off in spite of her plainness, she escapes to a neighboring kingdom and gets a job as a kitchen maid. She also falls in love, of course. It's such good book; so funny, so spunky, so everything you want when you're a bookish little girl who loves princesses, especially those who refuse to be locked up and saved. I actually recently reread it because I wanted to write something about it, but it didn't really work, the essay I was planning.
How about you? What novel or book of stories or poems have you read the most times, ever? Why?