I wouldn't say "not too rowdy/packed" fits the definition of "best place to watch the Sounders" -ideally, if you can make the games you'd want the next best thing, loud and rowdy. But I degress. Here's what I've found so far:

Downtown
-Fado's is the official gorilla soccer HQ. It's packed pre-game and they charge admission post-game, but during the game its oddly calm. This probably is what you're looking for.

-Kels sucks. Shitty service, over priced, not Sounders focused.

Ballard
-Bad Albert's is a cheap dive with decent food and ok screens. Meets your definition above. But if you want atmosphere...

-The Loft- finally, I justify the existance of a sports bar. Last week's game was PACKED -couldn't even stand- and all but one screen was on the Sounders (the other one was some basketball game).

-Snoose Junction dedicates their TVs to soccer, has good pizza, decent beer and recently Hard A. And they're a cool place, too.

-The Wine shop on 24th and 60th: super tiny featuring all local, all organic wines (and all expensive), it's pushing the itself as a Sounders venue. I think I'm heading here for the next game.

Those are all the places I've been to so far. I hope it answers your question. As the Sounders get bigger, I've noticed more and more places showing them, so have fun!