One of the great "what ifs" of recent American politics concerns what might have happened if McCain had won the Republican nomination in 2000 instead of George W Bush and gone on to be elected President. Obviously, 9/11 would still most likely have happened and there'd still have been an aggressive US response (since doing nothing would have been politically impossible), but I suspect an experienced military man like McCain - who, crucially, could hardly be more personally aware of the downside of campaigns going wrong - would have approached it rather more intelligently.
Politically, McCain and I are poles apart, but he was demonstrably a man of honour and principle right to the end. And anyone who could effectively sentence himself to five years' incarceration in appalling conditions with more or less daily beatings because he stood by his principles (famously, he could have been freed if he'd played the card that his father was a four-star admiral, but he refused to do so unless other PoWs were released too) was clearly a pretty formidable human being.
