Till we have Faces by CS Lewis is one of my favourite books. He retells the myth of Psyche from The Golden Ass, but in such an imaginative way, to represent the purpose of myth. I also love Abolition of Man, though not fiction, which seems almost prophetic sometimes.
There are great parts in Narnia, such as when the Lady of the Green Kirtle tries and convince the heroes in the Silver Chair, that her subterranean world is the only world, and Narnia but illusion. Essentially a Plato's Cave, but very well written. The Space Trilogy has some good points, too. All in all though, I much prefer Till we have Faces. His short essays and apologetic work, though not entertainment, are all worthwhile reading.
Not too much a fan of Tolkien. Read the Hobbit and the first Lord of the Rings book, but didn't resonate with me.
An interesting author to check out, another Inkling and close friend of both men and who influenced them as well, is Owen Barfield. His views on meaning and myth thread through their works. Narnia was dedicated to Barfield's daughter Lucy, too.