Obviously as a Tolkien fan myself (though Lewis was a better author) I prefer LOTR, but I feel bound to point out that Tolkien created his world and his stories and that was it, nice as they were, whereas everything Lewis wrote was always quite deep and intellectual, with something beyond the story.
For example, it is all very well saying 'Aslan is Jesus' but it;s not as if he designed it to leap out the page at you. Needless to say, no dramatisation or filming of his work ever caiught any of it, hence such films and programmes do his work a disservice, unlike LOTR.
And 'The Screwtape Letters' is one of the best darn books ever...