Vince Flynn's Mitch Rapp series is very good. Mitch is a CIA operative goes about conducting covert ops for the government. The early books about Kay Scarpetta Medical Examiner by Patricia Cornwell are very good, the books in the last few years, (her last 4 or 5 books) not so much in my opinion. Same for those by Kathy Reichs. For a British flair, the Inspector Banks series by Peter Robinson are good as are David Hewsom's Nic Costa Series about an Italian detective, though you get a lot of background story also. And Jeffrey Deaver's books are very good.
There are so many I'm sure you will get quite a list from the contributor's on this forum.
Oh and Richard Montanari's stories on 2 detectives Byrnes and Bolzano, in Philadelphia and Tess Gerritsen's Isles and Rizzoli books from which they get the T.V. show. Her last one "Ice Cold" is very good as was "The Mephisto Club". Michael Crichton's "State of Fear" was very good and I though fast paced.
Ah so many books so little time.