Well Tolkien and Lewis are always recommended, but I assume you want something new. Of course, if you haven't read LOTR or the Narnia books, do so ASAP.
I don't read adult contemporary fantasy (e.g. Warhammer, Forgotten Realms, Dragonlance) so I can't give any advice on that (except a few to stay away from). However, I do read a fair bit of children's fantasy and many of the classics I read have fantastical elements.
For children's fantasy, I'd recommend "Redwall" and "Guardians of Ga'Hoole". "Akiko" is a children's comic and book series that I also like. The comics are significantly more enjoyable than the novels and you can get them collected in graphic novel form. They don't have a lot of depth, but they're nice light, escapist reads. Great if you're a daydreamer like me.
For the classics, I'd recommend "The Faerie Queene" by Edmund Spenser, the "Divine Comedy" by Dante Alighieri and Paradise Lost by John Milton. All three have Christian overtones (although significantly less noticeable in Spenser, it could be read simply as a fantasy like Narnia), but this is back when Christian art was cool. They're told in verse and they require your full attention, but they're very rewarding reads.
Arthurian legend is also always good. Personally I'd reccomend The Story of King Arthur and His Knights and its sequels by Howard Pyle and le Morte d'Arthur by Thomas Malory (I haven't read this one yet, but it's the closest we get to the "official story").
Where sky and water meet,
Where the waves grow sweet,
Doubt not, Reepicheep,
To find all you seek,
There is the utter east.