The final book in the Space Trilogy,
That Hideous Strength, is my favorite in his fiction repertoire.
The Abolition of Man is my favorite in his non-fiction. Both deplore the abject corruption and death spiral of modern academia on the altar of scientific materialism and Nietzschen trans-humanism (the deification of man over God by self-styled intellectuals who think that good and evil are obsolete concepts).
What is so remarkable is that he predicted all of this back in the 1940s.