The middle school with the best reputation is Washington Middle School. It seems to be the feeder school for Garfield High (which has more National Merit Scholars than Lakeside) and it features the APP program, the Acceleriated Progress Program, a course of study for gifted children.
I raised my son in Seattle and I wish I could give credit to the public schools, but cannot. We had to resort to private schools. We also did not have a choice of schools that we could rely upon. If we had, I would have elected a middle school that feeds into Garfield.
I must concur with the Northshore comment, as I taught many years ago at Woodinville H.S., and it's a very good district.
Now Washington and Eckstein middle schools are going down. Washington middle school's App program has been replaced by Hamilton and Eckstein middle school's music program is now beaten by Hamilton. Hamilton is now probably the best middle school in seattle now.
No doubt, Eckstein Middle in Wedgwood, especially if your child is interested in music. Amazing music programs for orchestra, band, wind ensemble, jazz, and even vocal. Great Japanese program with exchange student programs, great academics. Check out one of their performances.
I think it depends on what you mean by best. Snoqualmie Valley schools are incredible in my opinion. The teachers truly care about the students, the parent participation is nearly 100% and the test scores prove it. Furthermore, if you want your child to learn about diversity, this is a great place. Alot of kids these days have so much, and it's hard to teach them that not everyone is so fortunate.