The general feeling among a lot of pundits is that if Clinton runs- and pretty much everyone knows she will- she's going to win the primary and then almost certainly the general election.
There's always room for an upset, though. If Biden decides to give the primary a go, things will become much more heated since vice president is not a position to be dismissed lightly. Plenty of Democrats don't like Hillary very much and would be happy to support another candidate if they thought someone else had a chance of winning the primary. Furthermore, Hillary isn't exactly a spring chicken- if she wins the 2016 election, she'll be sixty-nine upon taking office- as old as Reagan was in 1980. If, for whatever reason, she loses in 2016, her political career is over. (This might be tied to the fact that she's a woman- I don't think a man of the same age would give people as much pause, which is just silly, since women live longer than men do and stay sharper. But it's what it is.)
Other possibilities include Elizabeth Warren, darling of the Democrats' left, who- unfortunately- is very good at projecting an image of "liberal dingbat" everywhere but New England. She'd win against Santorum, maybe, but not against Christie or Rubio. Martin O'Malley, governor of Maryland, has also been discussed as a possibility, though I don't know much about him. I know RedKing has been betting on Brian Schweitzer of Montana as the dark horse.
There are some other possibilities, but they'll come out of the woodwork starting early next year, if they do. The candidate pool is fairly small, I think, because most potential candidates regard it as a fool's errand to try and stage an upset against Hillary. But hey, things happen- who'd heard of Barack Obama in early 2007?