Coming from the South (say SAN, LAX or SFO), the right side is the best. Great views of the cascade peaks all the way from nor. cal, through OR into SEA. On a clear day you'll be treated to spectacular views of Mt. Adams, Mt. St. Helens and Mt. Ranier as you approach SEA.

Coming from the east, it's really a crapshoot and depends on which way you get vectored. I would genreally say the left side is best (views of Mt. Ranier), but both are good.

Coming from Asia, I would say the right side. Nice views of the Olympics, Puget Sound and downtown Seattle. But again, it depends on how you are vectored.

The most consistently good views, based on my hundreds of flights into SEA over the last 20 years, would be on the right side when you are coming up from the South.

As other posters have said, no matter which side you sit on, the views are spectacular. It's why we put up with the grey days eight months out of the year. ^