I could have easily picked Harry Potter, but for the sake of repetition I won't.
This book (and series) just amazed me. The writing, and the character growth. We also had a queer* person as one of the main supporting characters (actually canon, revealed in the last book). It had plot, magic, and a love story, and Victorian-era girls running around causing chaos and general bad-assness.
I devoured this book and series in about a month. I liked it that much.