I'll address each of the candidates individually:
Clinton: She is leading in the polls right now by an extraordinary amount, but once people actually start to care about 2016, her lead will certainly decrease as time goes on. Claiming that Hillary is a lock because "omg she's leading in polls a year before the election" makes no sense. Opinions change over a year, people. Clinton does have numerous factors going against her, including the fact that she represents the democratic establishment, coming from the Obama administration. I didn't vote for her in the poll because a Clinton presidency would be even less progressive than what we have right now.
Warren: I would enthusiastically vote for Warren. I disagree with this idea that Warren will lose in a landslide because she's "far left" or whatever. Warren has a populist style that would appeal to working class voters who voted Romney over Obama.
Webb: Although Webb is better than Hillary, he's not going anywhere. He's terribly underfunded, as evidenced by his kind of pathetic 2016 website. I doubt that his exploratory committee will evolve into a full scale campaign, and if it does, it'll about as successful as Huntsman or Paul were in 2012. Again, Webb's populist rhetoric is the kind of appeal that could pull the white working class to the democratic party, but his anti-immigration stance is enough to push Hispanics towards the republican or stay home, depending on who the candidate is. I did vote for Webb in the poll, although in a hypothetical 2016 election, I'm not sure if I would support him. He is pretty progressive on most issues, but his immigration stance is unappealing. However, it is just a hypothetical scenario, and one that is unlikely to happen.
Sanders: He's more interesting. Sanders also has the same populist thing going on as Warren and Webb. In fact, this is evidenced by the 2012 election, when Sanders received a higher percentage of the vote than Obama in Vermont. Despite the fact that he's an Independent, he's been a hero for the left wing of the democratic party for awhile now. I find it hard to believe that he would receive much opposition from voters by becoming a member of the democratic party in order to run, although I'm sure that some of his primary opponents will try to take that angle. I would enthusiastically vote for Sanders over all of the other options provided in the poll.
Biden: Nobody with half of a brain would want Biden to run for the nomination unless they're looking a victory for the republicans. I don't dislike Biden personally, but the man is practically a gaffe machine and is attached to an unpopular administration.
O'Malley: Idk why this guy is even taken seriously. No one even knows who he is outside of Maryland, and he doesn't represent any faction of the party that isn't better represented by one of the better candidates.
Cuomo: Anyone who thinks this guy has a snowball's chance in hell of getting the nomination didn't pay any attention to his reelection campaign this year. (He essentially campaigned as a full-on right winger.) The man would be crushed in a primary, as he deserves to be.