As a Republican I could care less who the Dems run, but if were a Dem party official, I would think that Hilary would be their best bet just because of the fact that in reality, Democratic voters would really be casting their vote for Hilary AND Bill as a couple.
Personally, as a Republican, my choice for the Dem spot, if I'm asked who I would want in the White House if the Republican can't win, would be Joseph Lieberman. I think he is the least liberal and most honorable of the Democrats who have previously run for either Prez or VP.
If neither John McCain nor Rudy Giuliani is the Rep candidate, I think Hilary and Bill will be back in the White House. Here's why I believe that: Dem voters will jump at the chance to get BOTH the first lady president AND Bill as the first first uhhh, "gentleman".