I'm not one of those LotR purists who went raving mad with every alteration PJ and his team made to the book - some of them were okay, some were understandable and many were just bad. But one thing that really got my blood boiling was the way they reduced Gimli, son of Glóin, into a piece of mere comic relief - which even wasn't so funny. That I was really mad about.
But that said I think there is no competition here.
++ Túrin
It's an age old Finnish folk-story about Kullervo son of Kalervo which took my imagination already when I was very small and heard the story (the Finnish version that is) the first time. It's just one of the most tragic mythical stories there is, comparable in many ways to the story of Oedipus.
There's so much in there!
And one doesn't have to love the mythical hero as a person to see their literary, symbolic or philosophical greatness and deepness.