My reasoning: Obama might possibly have had a chance in the ridiculously early days when candidates were declaring, but the moronic self-fulfilling prediction of "electability" gave Clinton an all-but invincible imprimatur back when most of us were wondering if folks were seriously talking about this crap a year before the gaddam primaries. Giuliani and Romney are more of a toss-up, but I think the entrenched powers on the Right see Romney as a more familiar and amenable candidate, and that should swing the balance his way.
Edwards and Huckabee** have never had a serious chance. The only reason we hear about them is because the media knows that you don't sell fifty-some weeks of headlines that all say "Leading candidate continues to go through the motions; electorate is appeased".
*Not "Hillary". Yes, I know we had another "Clinton" in the office recently, but I trust we can all figure out who's who with some context clues. People managed to keep G.W. Bush and G.H.W. Bush distinct, after all.
**I'll concede that there is a chance that Huckabee could split the vote of some key demographic (presumably fundies) and thus play kingbreaker. Conventional wisdom says that his pull is social conservatives, which would primarily hurt Romney. I don't see this as probable, and if it seemed likely to the powers that be, I suspect he'd be promised VP or some other plum assignment in exchange for dropping out and endorsing his opponent.
Edit 08Dec2009: Not that it matters, but I feel the total and utter wrongness of this prediction in every specific is worth pointing out.