He pretty much created the technothriller genre with books like The Hunt for Red October and Red Storm Rising and created many great characters, chief among them Jack Ryan (Patriot Games) and John Clark (Without Remorse) - but the real heroes of his novels were the men and women on the firing line. Grunts, jarheads, squids, zoomies and Coasties, CIA operators and desk jockeys, FBI and Secret Service agents, Tom Clancy loved us all and wrote about us doing our jobs the best we could, sometimes under pretty shitty circumstances. Most of his novels fell into the Jack Ryan universe, and the later ones suffered from bloat and angst, but even so they were mostly good reads. My personal favorites, aside from the preceding, were The Cardinal of the Kremlin, Clear and Present Danger, The Sum of All Fears and "the other John Clark novel", Rainbow Six.
And the amazing thing about it was that he wasn't a former military guy, he was an insurance salesman who happened to be what the Japanese call a military otaku, fascinated with the men and the equipment and blessed with a mind that could make shrewd guesses about how it all fit together and went into action. He had a LOT of fans in uniform, at least in the junior officer and enlisted ranks; maybe fewer so in the flag quarters, because he was honest enough to show them as the politicians they sometimes evolve into.
There are many authors like him now, but he was the first, and he'll be missed.