what are his best books?
Splinter Cell was the GOAT
Red October is probably the best. I'm also very partial to Executive Orders and Debt of Honor (which will never, ever, ever be made into a movie in my lifetime.)
[Spoilers]Debt of Honor is about an American/Japanese war which is pretty well written, and ends with another Japanese defeat. Jack Ryan at the end of the novel is made VP of the US, and right after that, a Japanese airline pilot crashes a plane into the Capital killing the President, Congress, and the Joint Chiefs of staff making Ryan president. Executive Orders follows up on that, and has the aftermath, and a ground war against Iran which conquered Iraq and launched a chemical attack on the US.
President Ryan launches a cruise missile at the Iranian President's house on live TV to end the war and warn everybody what happens when you piss off Merica. [/spoilers]
I enjoy those two greatly and they are very well written, but those start Clancy on his political soapbox where he gets extreme with his views. His later books are also pretty much ghost written and sprinkled with extreme political views.
The start of Pres Ryan is like a moderate republican. Later on in his books, it turns into Tea Party/Fox news republicans.