I'm going to be a bit cheeky and say Pulp by Charles Bukowski.
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.
More of a spy novel series, but the Charlie Muffin books by Brian Freemantle.
I'm going to cheat slightly and say the Perfect Dreyfus series by Alastair Reynolds. Even though they're set in the distant future, they still follow the detective novel formula in my opinion.
