My favorites-

Anything by Nelson DeMille, especially the John Corey series.
The Alex Delaware series by Jonathan Kellerman, or anything by him really.
Stephen Hunter is pretty good too. My top two by him are "Black Light" and "Point of Impact," which was the basis for the movie "Shooter." The book is far better, as per usual.
Clive Cussler's older stuff. My favorite of his is "Sahara," again far better than the movie.
Have to agree with tmleadr03 about the late, great Robert B. Parker. But I prefer the Jesse Stone series.
I do like the Scot Harvath series by Brad Thor, as has been mentioned.