I thought these numbers were pretty interesting. They come from a YouGov Poll done between December 6th and 8th of a random national sample of 1,000 adults, with a margin of error of about 4 points. The survey didn't ask who the respondents support for president, or who they would vote for. Instead, it shows the percentage of the population that view each of those candidates favorably (in both parties).
Hillary Clinton is viewed much more favorably right now than any other candidate (51%), but Joe Biden finishes in a pretty strong second with 43% (about 8 points behind Clinton). Elizabeth Warren was viewed favorably by 31%, and no other Democrat could top 22%.
None of the Republicans was viewed as favorably as either Clinton or Biden. Romney was the most favorably viewed at 35% (16 points behind Clinton), while Christie and Huckabee were viewed favorably by 33%. Bush and Paul finished at 31% favorability, and all other Republicans scored less than 30%.
It's still a long way to the 2016 election, and we haven't even had the primaries yet -- so these numbers don't tell us who will win in 2016. But I think they do show the Republicans have a lot of work to do before they can be competitive with Clinton.
The survey also showed the party ID percentages of Americans (see chart below). A plurality of Americans now call themselves Independents, and all demographic groups but four showed that. The four groups (women, Blacks, Hispanics, and those over 65) had a plurality or majority for the Democratic Party. No group showed even a plurality for the Republican Party -- and only one group showed Republicans fairly equal to Democrats (Whites, who were split at 28% Democratic and 29% Republican).
The 2016 election will be won by the party that can appeal best to Independents, but the Democrats currently have a small advantage.