I can never pick just one, but here's a few I thought of:
Howl's Moving Castle by Diana Wynn Jones
Howl's Moving Castle is one of my favourites for it's approach to romance, I'm also a sucker for a fantasy novel.
This book also happens to be the one the Studio Ghibli Movie is loosely based on.
Earthsea by Ursula K. Le Guin
a Fantasy series with some darker themes.
It also has a Studio Ghibli film that was loosely based on it.
The Squire's Tales Series by Gerald Morris
a new take on the age old tale of King Arthur and his knights.
This series is the first one that got me into reading and will forever have a special place in my heart.
(an edit because I only just read the rest of the blog introduction, apologies)