I know that this is a pat answer, but probably C.S. Lewis. After that, maybe George MacDonald, if he counts as a Christian fantasy writer. Lastly, Frank Peretti, who is good occasionally.
Why? I loved the imagery of Lewis and MacDonald, and I love Peretti's angels. Further, there weren't that many "Christian" fantasists to choose from, during the period of my life when I was seeking out Christian works.
As for "best" novel, I think probably Lewis's The Lion, the Witch, and the Wardrobe still reigns supreme in my heart. After that, his Space Trilogy.