Well, truly EASIEST would probably be plane. XD But cost-effectively.... I would probably say Greyhound. I have never made that trip on Greyhound, obviously, but I know they only charge $30 for a trip from Portland to Seattle, and that trip would be a bit more expensive if you were going in your own car, so theoretically it might be good. I'd check around though, train might work just as well.
Then again, I'd compare those prices with airfare; sometimes on long-distance traveling they end up being pretty close! :twisted: