I think that the best thriller ever written is
Frederick Forsyth's "The Day of the Jackal".
Running it pretty close, though, is Murray
Smith's "The Devil's Juggler" This has
everything - the Secret Service, the SAS,
the first Gulf War, the NYPD, a Columbian
drugs cartel, and an excellent plot tying them
all together. It's a real page-turner.
And I agree with those who recommend Lee
Child's Jack Reacher novels. If you like thrillers,
and you've never read any of these, I envy you
the reading pleasure you have to come.