One of my all-time favorite fantasy series is Guy Gavriel Kay's The Fionavar Tapestry.  It has so many elements that connect it to traditional mythology and epic fantasy, so that it feels familiar and comforting.  But Kay also does a wonderful job of making the comfortable just a bit strange, so that it's not always easy to read, or simple to understand--it's the perfect example of a series that is firmly rooted in tradition, but pushes the boundaries of that tradition in exciting, thought-provoking ways.