I think Hillary should either choose Elizabeth Warren or Julian Castro as VP.
Warren would clearly be the most energizing pick for Hillary and for the campaign. Selecting Warren would add star power to the Democratic ticket that would greatly enhance the historic nature of the candidacy. TBH she is the most commanding figure, other than Bernie Sanders, in the progressive wing of the Democratic Party.
Julian Castro as a vice presidential pick would provide the Clinton campaign a younger political figure and reminder of diversity. He comes from an immigrant family, and he could make sure that a high level of Latinos support the democratic ticket. This could be extremely helpful in states like Florida, Nevada and Colorado, where Latinos are a major part of the population. He could also help with young voters given his age. I think either selection would be good.