Wow, thats a tough choice- he has been MY favourite author for God knows how long. I have read and reread his works for God knows how many times!
Back to the topic in question- I guess... it has to be Without Remorse- you can almost taste the suspense and tension in the manner that Tom Clancy penned it. Comin up close would be Red Storm Rising- I always wondered if Hollywood would ever make a movie out of it...