I'd say it will be Senator Kaine of VA. Respected, likeable, swing state, speaks Spanish fluently. It won't be Senator Warren and Secretary Perez is lacking gravitas. Number two doesn't mean as much for Mrs. Clinton. For Donald Trump, it's his first test of Presidential decision making and very important.
My DC insider says the smart money is on Newt, but I still think it will be Governor Martinez.
Hillary Clinton’s top five vice presidential picks | TheHill