Right now my favorite VP candidate with Hillary would be one of the Castro fellows.
Then they could be pretty well assured of carrying both Texas and NY, 2 of the biggest.
If no Hillary - and I'd mourn her loss - some combination of Warren/Castro/Booker/??? There are others I like a lot but electibility must be considered. I used to want Cuomo but he's been getting some scandals lately (true or not) and you wouldn't want him for Hillary's running mate because they're both from the same state. Pragmatism must rule the heart if we want to win as many places as possible. For instance, no matter how much I adore Kucinich and Bernie Sanders, I wouldn't make a $5 bet on either one to win.
For me that's what matters most - getting the strongest candidates within a reasonable political circle so that we can WIN. Otherwise you'll find another bunch of Repukes sitting in Congress laughing their arses off at us while we shiver out in the cold with nothing but our purity blankets to huddle under.