A) definitely a Democrat.
B) preferably a Woman.
C) preferably one who'd be tough on immigration, but left-wing on other social topics (which virtually fits every one of them).
So, who would that be ? I assume Gillibrand, Klobuchar, Warren etc. are all quite open-border ? If there is no viable Democratic woman fitting this criteria (immigration), I will eventually settle for one of them.
Klobuchar's the closest to a policy wonk out of the three. I don't think immigration is part of her big issues and you can expect her to balance some enhanced smart border security with amnesty for those already here. She's the most moderate of them.