It's hard to pick just one, but 'The Spy Who Came in from the Cold' is often considered a standout. It's a classic of the spy genre with a compelling plot and memorable characters.
In my opinion, 'The Night Manager' is the best. It has a great balance of action and character-driven drama. The plot is engaging and the settings are vividly described. It's a thrilling read that stays with you long after you finish it.