Tulsi Gabbard would surely be the best candidate by a mile to stand against Trump.
The VP on the ticket is going to have to be a woman. Gotta bring in those female voters. Gabbard would be good but Hawaii is a blue State anyway. They really need a woman (esp if a minority female) from a purple State to help draw that State into the blue column. Klobuchar would pull in MINN.
Whether it's Biden or Bernie, the old dude is likely to die in office (or lose it mentally), so that VP is going to likely become Pres.