First, let me tell you why I don't think Rudy Guliani, Mitt Romney or Fred Thompson are the best choices.
Romney is a landslide loser waiting to happen and will never beat either Hillary Clinton or Barack Obama.
About Thompson, I don't think the American people is in a mood to vote for another actor to be their president in a time of war. I don't think people want somebody like Fred to be their president after having Bush for eight years. Look I like Dubya, but the mainstream media has characterized him as an unqualified idiot, and most Americans seem to agree with that viewpoint. The last thing they need is another candidate who they perceive as an unqualified "Bush type" candidate.
About Guliani, I like him, and I agree with his social views and his stance on national security. But he seems to be the type of candidate Hillary can beat. He makes hillary look like the "Family Values" candidate since his personal life is even more messed up (3 marriages) and is almost as liberal as Hillary, which will cause problems with the GOP conservative base. And like Hillary, he seems to me to be a guy with a shady past (that we still don't know about yet, but will find out once he becomes the nominee.) He may be the 9/11 candidate, but the US haven't been attacked since 9/11, and domestic security is not resonating as much as the issue of the Iraq war--where his stance is not as forceful as mccain's.
So why McCain?
1) Having McCain as the GOP nominee will be good for (the future of) Iraq--which is the single thing that is driving Bush and Republican numbers down. I think Iraq will be the main issue in 2008, and you need it's most credible supporter (the guy who best understands what's at stake in iraq) and the most competent person to takeover from the Bush administration's early mishandling of the war.
2) Having McCain as candidate means Hillary will have to be more careful about dissing the military and the troops in Iraq. And she may even be forced to abandon his anti-Iraq war stance if "the Surge" continue to show better results. McCain's candidacy should give GOP candidates in the Senate and the House enough "backbone" to withstand some of the attacks from their liberal counterparts. And if Iraq succeeds, who is more deserving to receive the kudos for believing in the troops than McCain?
3) Unlike Guliani, it's not as easy for Hillary's camp and the Left-Wing attack machine/Liberal MSM to dig up dirt and smear a war veteran. It helps that this guy was the media darling a few years ago for criticizing the Bush admin's handling of the war. Hillary may be the media's favorite presidential candidate, but McCain I believe is the MSM "favorite Republican."