I would take a look at the weekly stocking report...I'm sure some of the faithful out there will disagree with me, but if you are relatively new to Flyfishing in the area you might as well hedge your bets!

Here is the site:
http://www.wa.gov/wdfw/fish/plants/weekly/

As far as the best lakes in the Seattle area, kinda hard to say...the best I have found are over on the Samammish Plateau or in South King County. If you really want to stay near Seattle, try Angle - I heard it is fishing well these days.

Don't forget the woolly buggers and carey specials...and if you find a good dry fly pattern, let me know. I still can't crack the code on my favorite lake with the drys.