There's a big, squishy place in my brain that must be fed and it demands only the finest in YA literature. Sabriel, published in 1995, was my introduction to fantasy (a genre which would, in middle school, become a popularity-handicapping passion). I was 10 or 11 the first time I read Sabriel and have read it 5 or 6 times since. Nix's success in Sabriel's world-building and character construction easily matches that of Philip Pullman's much more popular His Dark Materials trilogy.
On the cusp of the impressionable, tumultuous time that is adolescence, Sabriel made me feel like an understood, accepted human being just by the virtue of it's indomitable female lead. Sabriel does not waver in the face of trial, does not bend to will of boys (or girls!), and seems to contain a font of endless bravery.
I LOVED this book. I think I started reading Lirael but didn't finish or get to Abhorsen...I've always kind of wanted to revisit them. Sabriel is one of the few positive female role models I remember from my youth as well.