King County International Airport, or Boeing Field (BFI) as it is commonly known, is the largest business and general aviation airport in the Seattle area. If you are flying your Gulfstream or Challenger in to Seattle, this is the place you are likely going to be landing.
There are a few scheduled services in and out of this airport, which include Kenmore Air Express and cargo flights with UPS & DHL (FedEx is based at SeaTac). The major traffic at this airport comes from general aviation, business jets via the Fixed Base Operators (FBOs), and Boeing test flights.
Because of this diversity, BFI is a great place to go aircraft spotting.