For the stuff that hasn't been made into movies, The Cardinal of the Kremlin is probably the best "military espionage" book done by Clancy. I'd also recommend Debt of Honor, and, especially, Red Storm Rising. I put RSR at the end because it's not really focused on espionage stuff; it's more like a war simulation written in book form. That said, it's my favorite Clancy book that hasn't been made into a film.
I wholeheartedly recommend Patriot Games and Hunt for Red October, but those have been put to film. The books are better than the films, though. Clear and Present Danger was a good one, but a notch below the two above. The Sum of All Fears -- why did they have to do such a horrible job on the movie? -- is good, but paints a much different picture of the main character (Jack Ryan) than the other books do, and it has a feel to it that keeps you a bit more creeped out and on edge than the other works I've cited do.
The others in the "Ryan Series" -- Executive Orders, The Bear and the Dragon, and Red Rabbit -- are decent reads, but you should start with the others first. IMO, avoid anything written after the Ryan Series, i.e. the Op-Center series and others. Those are primarily written by other authors are not of the caliber of the other works.
Dave