If it was today, looking at Democratic poll numbers, I don't think its even close, Hillary wins the Democratic nomination.
Watch for Hillary to snap up all the top Democratic talent off the bat, much like what Bush did in 2000. I think the only chance for a different democrat to win it is to throw their hat in the ring early, and then go after the best campaign managers the Democrats have.
I don't think she is as polarizing as everyone thinks within the democratic party. Most of the other top candidates being named (Kerry, Gore) are equally, if not MORE polarizing due to people viewing them as losers, not winners. They wouldn't get the same type of monitary support that they did in their first campaigns. Especially Kerry, who left something like 13 million in the bank rather then spending it in Ohio, which pissed off a lot of the big campaign contributors for the Dems.
As for the Republicans, I think its a two man race between Rudi Giuliani and McCain if they both run for it. It'll be interesting to see what Rudi does to try and get his name out there, he hasn't been in the news a lot since he left his post as the mayor of NYC. McCain has a good advantage with incumbency in the Senate.