Top 5 candidates (not necessarily in popularity order):
1) Joe Biden- Nearly an octogenarian. Comes off as creepy to many. Is a corporate whore (first fundraiser was with a Comcast executive). Comes off as genuine but doesn't articulate policy well.
2) Bernie Sanders- Nearly an octogenarian. Moderates won't vote for a socialist. Has dragged many candidates so far to the left for the Democratic primaries that they'll have a hard time tacking right for the general election. The demographic he appeals to the most doesn't tend to vote in large numbers.
3) Elizabeth Warren- Articulates policy proposals better than any other candidate. But she's dragged herself down with the Native American stuff (eternally "Pocahontas" in conservative circles) and might not be seen as trustworthy by moderates who also see her policies the same way they see Bernie's.
4) Kamala Harris- Hard lefties don't like her because she's been involved in law enforcement. And they don't like law enforcement. To everybody else she's just a crazy from California who will say (or not give a real answer to) anything. Not genuine. Not workable on a national level.
5) Mayor Pete- My favorite among the popular candidates. Smart, articulate, veteran. But also very openly and proudly gay married. I have absolutely no problem with it, but Republicans would do everything they could to make the public feel uncomfortable about him smooching his husband on the campaign stage. Memes, photoshops, nicknames, etc.