I second Robert B Parker’s Spenser novels, some of the funniest dialogue in a PI series ever.
Zoe Sharp’s Charlie Fox books are terrific, very intense and Charlie Fox is a wonderful heroine.
John Connolly’s Charlie Parker books action-packed and grimly compelling with a spooky, supernatural edge.
Dennis Lehane is better known for his gangster novels, but he started writing with a series featuring a Boston detective. Ignore the film, it doesn’t do the Patrick Kenzie series justice.
Also, although they might seem an outlier, Dick Francis’s Sid Halley novels are worth a look. All of Francis’s heroes share a kind of steely determination which is not unlike Reacher and Sid Halley is a good character. I think Francis, while popular, is over-looked sometimes when thrillers are discussed. Avoid the later books written by his son, I think they’re dire. I’m not looking forward to Reacher being passed over to Child’s brother-in-law either.
Peter Temple’s Jack Irish books are terrific. Everything Temple wrote is terrific, but the Jack Irish series tread the “down these mean streets a man must go who is not himself mean, who is neither tarnished nor afraid. He is the hero; he is everything.” trope pretty well.
Carol O’Connell’s Kathy Mallory books don’t get enough love. Kathy Mallory is a sociopath on the side of justice, but maybe only just. The books are quirky and twisty.
You might also try Lawrence Block’s Matt Scudder books about an ex-hit man. Block is such a good writer.