Hmmm...I don't read too many books that are in series, but I'd have to say 3 authors come to mind as amazing, all in the Detective field: (Which is curious, as this is perhaps about 10% of what I read overall, by genre)
1. Raymond Chandler and his Phillip Marlowe novels. (The Long Goodbye, The Big Sleep, The High Window &c) Easily some of the snappiest, crisp prose ever written, as as a detective Phillip Mawlowe is witty, mouthy, scrappy and never in much favor with criminals, the police, or even, at points, with his own clients.
2. Ross MacDonald and his Lew Archer novels. (The Way Some People Die, The Barbarous Coast, The Doomsters) Easily the heir to Chandler, with more of a literary punch. Blunt action, nuanced psychological derail.
3. The Inspector Montalbano Series, by Sicilian novelist Andrea Camilleri. The author is amazing, must be 90, a chain-smoker, and still writing. Funny, with a lot of fun poked at the bureaucracy, and set in an exotic, beautiful Sicilian landscape.