Personally I prefer
Hilda Solis -she has a long history of public service, serving in the California State Assembly, Congress, Labor Secretary in President Obama's first term, and now as Los Angeles County supervisor. She has developed deep ties to the environmental, labor, and immigrant rights movements throughout her career, and would go a long way in reconciling the more progressive elements if and when Hillary eventually wins the nomination.
As an added bonus, she was and is a Clinton loyalist, having been a strong supporter of her candidacy during the 2008 electoral cycle until Hillary finally conceded to Barack Obama -she is someone Hillary can trust and depend on. She is experienced, unquestionably qualified, and more than capable of serving as President of the United States should anything happen to Hillary.
And with the GOP seemingly intent on committing electoral suicide next November, I think Hillary can safely afford to pick a woman as her running-mate, and who better than Hilda Solis to run alongside her?