I like Warren, Wyden, and Ellison, but I would vote for Harris.
I know a prediction wasn't part of the question but this is how I see a Harris/Wyden/Warren/Ellison/Conley primary playing out. Harris keeps most of Hillary's core base (winning among women and minority voters), while be much more popular among the youth. Warren manages to squeak out wins in educated pockets in the Northeast, and Ellison narrowly wins his home state of Minnesota. Conley makes SC close but still loses it to Harris, so he drops out early. Wyden wins the states Bernie did in the Pacific Northwest, Plains, and Mountain West, but can't hold onto crucial Midwestern states like Michigan and Wisconsin. Harris' big advantage in urban areas nationally leads to a convincing primary win for her.
Kamala Harris (D-CA)Ron Wyden (D-OR)Elizabeth Warren (D-MA)Keith Ellison (D-MN) (gray on map)