Police Detective mysteries: Try the Harry Bosch series by Michael Connelly. Start with The Black Echo and read the series in order. The detective ages in real time so in each year's book he is a year older, all the way to retirement.
Police Detective, thriller style: Try John Sandford's "Prey" series or his Virgil Flowers series. Most are written in the style of a thriller, with chapters alternating between hero/villain POV.
Gov't Agent thriller: Try Nelson DeMille's John Corey series. Start with Plum Island and read in order.
Private Eye Mysteries: Try some hardboiled classics from Raymond Chandler (The Big Sleep, Farewell My Lovely, The Lady in the Lake) and Dashiell Hammett (The Maltese Falcon, Red Harvest). The Elvis Cole - Joe Pike series by Robert Crais is great for mystery, action, and wise-cracking humor. For a new voice, try IQ by Joe Ide (and the follow-up Righteous, which I haven't read yet).
Hope you find some you like!