It's not Seattle, Tacoma or Pullman. It's not Vancouver, Everett or Olympia...

So what was their criteria?

Political donations.

In the 2015-2016 election cycle Bernie Sanders is considered the most liberal candidate, and the number of people (percentage) donating to Bernie far outweighs everywhere else. This is according to a political junkie website.

So now you know.

maps.google.com