Mere Christianity is probably his most well known theological work, and it's not bad, though not my favourite of his. A few very impactful sections though. If you haven't had much prior exposure to apologetics, this I'd stronly recommend this, if you have, then it gets a moderate recommendation, as you've probably seen a lot of what he talks about.
The Great Divorce was great and pleasantly short. One metaphor in particular, about addiction to sin, by itself was worth the price of the book. The ideas he presents about heaven and hell aren't so much about Truth in the literal sense but about provoking and inciting our thoughts about what may be True. Before this book, my image of heaven was quite dull ... harps, singing, and monotony ... this prompted me to see heaven as much more.
The Screwtape Letters is a very creative collection of letters that a demon (Screwtape) writes to his nephew, a Junior Tempter. A fun read with some unique insights into humanity.
I read The Chronicles of Narnia often when I was younger, but in a recent re-reading I was surprised by the depth in them. The university I went to (a public school) had several CS Lewis literature critique-type books, including a couple on Chritian metaphors in these books ... I had thought there was the occasional metaphor, but was delighted to see that these books were packed with theological truths.
I would also recommend the The Abolition of Man, which discusses the evils of moral reletivism, particularly in the context of education.