Namely: Martin O'Malley and Jim Webb. Both of them are smart enough to know their chances against Hillary, absent an enormous cash infusion from somewhere plus some SERIOUS VIP endorsements, are minimal. At best they can jockey for VP, and even that is a prize that may not necessarily be determined by how much of a challenge they mount against Hillary. If she, or her team, perceives that a particular challenge bruises her enough as to endanger her chances in the General Election, she'll probably look elsewhere for her VP (Castro, who has been mentioned a lot in speculative articles, would be a logical choice). Webb could probably do the job. O'Malley definitely could do it, and it would set him up for a presidential bid 8 years later, while giving him foreign policy experience he could tout in a campaign. Same goes, of course, for Castro. Republicans tend not to know a lot about the world when tested (e.g. Romney, W.)

Webb is 66, so he is probably out for a run at the Oval Office at age 75. VP for him would be the end of the line. Not that it has to be a bad gig. Biden has acquitted himself admirably in the job.

There are others, but none of them have expressed the slightest interest in even forming an exploratory committee.

I think all potential and declared candidates, both friend and foe, are all waiting for Hillary or someone close to her to make that one big stupid mistake that will cause her perceived inevitability to crack. If that doesn't happen, don't look for too many Democrats to pound their heads against a brick wall for long. In politics, it's wiser to get on a moving train than to try to stop one with your bare hands.