I saw the Pats play in SEA this year, the week after I went up to see the Timbers. Between the two Seahawks have nothing on Sounders fans. They were mostly pleasant (but I wasn't wearing Pats gear and had a 4 year old with me). Sonics fans really are taking the cake on the twitter wars though.
I also saw the Mariners play the Red Sox, and while at least 50% of the stadium and town that weekend also seemed to be Sox fans, it was a very pleasant experience in a nice park. So they have that, I guess.