I've read all the Jack Reacher books and liked them all. Have read, I think, most all the John Corey books as well.
There is a series of books by John Connolly - the Charlie Parker series - about a detective that loses his family and the path he follows. Maybe 15-16 books? The first book is 'Every Dead Thing'. The general theme progresses from serial killer stuff and slowly bends into a bit of supernatural and good vs evil. I like them all. Best read in order as new characters are introduced, but ok as single books as well.
Another short series would be from Baldacci - the Will Robie series. Not a detective, but a govt assassin with conscience. I think there are 4 books? The Innocent, The Target, The Hit, End Game are what I seem to remember but there may be another. Again, probably best read in order.