Two mystery/thriller series I've read recently that have good narration (note that both of these series contain a fair amount of violence, some of it quite graphic):
Randy Wayne White's Doc Ford books (about a geeky-looking Marine biologist in Sanibel, FL who used to be (or maybe still is) an NSA agent as well. He has a very kooky Florida-esque bunch of friends & tends to get involved in dangerous situations that he is drawn into when helping those friends, or friends of friends. Very good on the atmosphere scale; lots of old Florida references and descriptive language evoke the sense of place, and provide a bit of comic relief. They are set mostly in the 1990's, though the time frame is intentionally vague. Get the versions narrated by George Guidall, who does all of the voices himself, doing a nice job of differentiating the characters while not over-acting. (Currently 26 books in the series.)
The Billy Boyle series, by James R. Benn, features a just-a-tad-shady young Boston cop who is slouching through WW2 as an aide-de-camp to his Uncle, General Eisenhower. Ike has turned him into something of an internal-affairs investigator. The series starts just after Dunkirk and works its way all the way through the war. Each book centers around investigation of an obscure situation that happens adjacent to a major military campaign; most of the situations are based on something true, though perhaps embroidered for dramatic effect and to place the character there. The events take place in a host of different European (and in one case South Pacific) locations. The narrator is Marc Vietor, who does a good straight read, without making a lot of effort to differentiate voices. (Currently 15 books in the series.)
Also, a very good non-fiction title I enjoyed on audio is The Woman's Hour, an account of the machinations surrounding Tennessee's ratification of the 19th Amendment, which gave women the vote in the US. Both sides used every trick in the book to influence the vote, and the story is fascinating. The narrator is Tavia Gilbert, who does use accents to try to differentiate quotes from different participants.