Some of my favourtie series are, I like travelling through reading, are
Connelly – Bosch & the Lawyer ones – Los Angeles
Donna Leon – Brunetti – Venice
Michael Dibdin – Zen novels, all over Italy, can be a bit whacky.
James Lee Burke – Robicheaux – New Orleans and Louisana
Nesbo – Hole – Oslo
Rankin – Rebus – Edinburgh
Robert B Parker – Spenser – Boston
Mankell – Wallender – Ystad
James Elroy, Elmore Leonard and George V Higgins are also worth reading. Carl Hiassen for a comic thriller.
Others from late 20th century:
Simenon – Maigret – Paris
Freeling – van der Valk – Amsterdam
van de Wetering – A trio of Police Officers – Amsterdam
Ed McBain – 87th Precinct – New York
The Classics:
Sherlock Holmes, Dashiel Hammett’s novels, Raymond Chandler’s, James M Cain’s, Chesterton’s Father Brown etc.
I could go on and on.