Hmmm. Not really a true "detective" novel and more of a crime novel, but L.A. Confidential by James Ellroy is probably one of my top three books of all time, with the entire L.A. Quartet taking a fair chunk of my top ten.
But my interest in Ellroy got me down a rabbit hole of classic noir detective fiction and my favourites from THAT era would have to be Poodle Springs and Farewell, My Lovely by Raymond Chandler. I'm pretty fond of a lot of Elmore Leonard's work as well. Dashiell Hammet for some reason never really clicked with me, nor did any of the Mike Hammer books.