Not a Democrat, but I chose Clinton. She has the best chance to win the general election and can preserve Obama's achievements. I like Sanders, but there's no point in nominating a socialist if they're going to lose. Even if he did win, the House is controlled by Republicans and there's the chance that they could narrowly re-gain control of the Senate as well.
In that case I think it would be much wiser for Sanders to stay in the Senate where he can do the most good and for Hillary to hold the Presidency rather than risk the Republicans undoing all the progress made over the last eight years.
Hillary, because I don't want to one-up the 2010 Delaware senate race on the national scale.
Or another 2000-style debacle.