I worked for a couple of years out of Office Nomads, and love the community there. The Capitol Hill location is perfect for people looking for access to that area. (No reasonable parking in the area.) Office Nomads kind of expects everyone to work together and turn to each other for resources, so it is a great place for a solopreneur looking to network.
Impact Hub is Pioneer Square is newer, and excellent as well. The space itself is more varied than Office Nomads, and generally less populated. The people at Impact Hub seem to be mostly working on their own stuff, and I have not experienced much inter-office collaboration on projects there.
WeWork in South Lake Union is an interesting mix of startup companies in glass-walled "cells", and some consulting solo entrepreneurs working from common desk areas. This space is very active. There is always a LOT of stuff going on at WeWork, and it is an energetic space. But it is less focused on networking between members, and more providing the infrastructure small startups need to get rolling.