Bernie Sanders.
Bernie has a very good chance to beat Trump and many other Republicans.
In a national election, Bernie's democratic socialism is nowhere near as relevant as the fact that his gun policy is nuanced, his foreign policy is much closer to the center, and he has a reputation for integrity and honesty.
O'Malley is actually a very fine choice, but Sanders will bring a new perspective that is needed to the White House. The rich and corporations have had their interests represented at the cost of everyone else's for too long.