There's no single work I could choose for this man - one of the greatest minds of the 20th century.
There's a sci-fi trilogy he wrote as a result of a challenge with Tolkien. (Tolkien was a member of the Inklings. Even though he was a Catholic, when Lewis became a Christian partly at his prompting, he became an Anglican.) The Tolkien got time, which led to the Hobbit and Lord of the Rings. Lewis got space - and we got The Space Trilogy.
He also wrote a children's series - Narnia. He also wrote The Screwtape Letters, from a Senior Devil to a Junior Devil.
Mere Christianity is his best-known work on his faith, but the one I find most intriguing is Surprised by Joy.
Autobiographical, It takes its title from an American lady (Joy) he became surprisingly smitten with. It wasn't "love", but when she was about to leave England to return home, he married her to avoid the necessity for that. They continued to live separate lives until she got sick. Remission saw him realise they were in love so a second marriage was followed by a far too short time of "normality" before she became ill and that time, died.
Many other works came out of his great mind - don't let my little list restrict your enjoyment of one of the greatest minds of the 20th century,