It depends. I don't think she has any real competitor. I like O'Malley a lot but he's probably waited too long getting his campaign started and funded to make a dent in Hillary's lead.
It'll take Hillary floundering to make it a race. If nothing unforeseen happens, she'll get the nomination and then it'll be interesting.
On the record, as of 04/24/2015, I don't think she'll beat the GOP's nomination, whether that be Rubio, Walker or Bush. She's not getting the same excitement I thought she would from the Dem base. I think the GOP'ers will come out to the polls to support their candidate - and I think the Dems will simply sit this one out. (If she's the candidate.)