If you're into fantasy, you've probably already watched it... But I'd say The Golden Compass.
I was a HUGE fantasy reader when I was around 12-13, unfortunately I can barely remember any of the books I read back then, just that I'd pick the thickest book in the library, and aim to whizz through them.
However, The Golden Compass was one book I didn't forget - and I totally loved the movie too!
This is going to sound a bit geeky, but I was good friends with our school librarian, and she used to reserve fantasy books when they first came in so that I could read them before anyone else :-)
That's how I ended up getting into Harry Potter before it was cool (then it became mainstream and everyone ruined it).
Anyway... Back to your question... I think The Golden Compass is a great movie.
It's based on a book of the same name (in the US), but in the UK it was known as Northern Lights.
The worst part about the movie is that it was filmed with a sequel in mind. Unfortunately, that project has been put on hold - so you don't get the sense of closure. One theory is that this is due to pressure put on Hollywood by the Catholic Church!
Anyway, the book is only the first part in a trilogy called His Dark Materials, so you can follow up on Lyra's adventures by reading the following two books.
And finally, here's a trailer: