Well, it's pretty obvious that McCain will be the Republican nominee, so the obvious question is which Democrat has the best chance of beating him. I honestly don't know, but my current analysis follows:
1. Media Coverage
The Media (MSM and most of the blogosphere) love both Obama and McCain. This means that, for possibly the first time in 20 or 30 years, we could expect fair coverage of both candidates (i.e. negatives will be minimized).
If Hillary is the candidate, we can, of course, expect her to be thoroughly trashed no matter what she says or does and no matter what McCain says or does.
2. Debate Performance
O v. M
Visually, we will see a tall, young, vigorous man vs. an older, shorter, weaker man.
In a debate between these two, both men have to be careful how they attack. If McCain focuses too much on Obama's youth, he risks raising the specter of his own age.
McCain must also be careful not to say anything that could be interpreted as racist. (This is less likely for McCain than it would be for Hillary because there is no presumption that McCain would play the race card.)
But Obama must also be careful about calling McCain too old, or implying that his experience and judgment are irrelevant or flawed. Whatever else McCain's opponents may think of him, they know he is a war hero. Just as McCain must worry about being perceived as "dissing" Obama, Obama must avoid the use of his "Hillary-dissing" strategies: looking down his nose, smirks, etc.
H v. M
Visually we will see a short, middle-aged woman vs. a short, older man. They will appear, more or less, evenly matched.
McCain must worry about talking down to Hillary or treating her as a lesser rival because of her sex. He will lose a lot of woman if he dismisses her experience.
Hillary, I suspect, will not commit any "perception" errors. She has known McCain for many years, she knows about his temper, she already knows how he treats her. That gives her an edge over Obama.
3. General Election
If there is a terrorist attack prior to the election, it will favor McCain. But which Democrat will it hurt less? I think Hillary, in part because she did support the vote to invade Iraq and because of her experience on the Armed Services Committee. Obama would be at a disadvantage because McCain could paint him as a peacenik who obviously doesn't know how dangerous the world is. (The fact that a second attack occurred under the same Republican President as the first attack will not hurt McCain. Why, I don't know. But it won't.)
Otherwise, I think either Obama or Clinton has a good shot to beat McCain simply because they are Democrats. Hillary would have negative media coverage while Obama's would be good. (See #1 above.) Obama is, of course, the more uplifting speaker, but there's the chance that during the campaign, his speeches could wear thin. And, as in the debates, he has to avoid the flip side of the race card.
Still, I don't think one can reasonably predict which Democrat would do better against McCain.