There are several airports in the Seattle area, but “Sea-Tac” as its called is the largest one. Unfortunately it does not have any sort of official viewing area – it was closed a few years ago to make way for a new runway.
There are two spots where you can watch planes around the Seattle airport. S. 188th Street actually goes under runway 16L/34R and you can pull off on the shoulder after the runway bridge. If planes are landing to the north you’ll see them fairly close especially on approach to the west runway (16R/34L) as they will fly over this street. I did not encounter problems with police while doing this. It is best to park on one of the side streets such as 16th Ave. South as 188th St. has limited space on the shoulder.
The other area is best if planes are landing to the south (runways 16L & 16C) and is the parking lot of a waste water treatment plant located next to the Seattle Airport Employee Parking Lot. It is located off of 24th Ave. South and S. 146th St. as shown on the map. Also, if you continue west on S. 146th St. to the end there will be a turn around area at the baseball diamonds. This will give you a closer view of traffic approaching runway 16R. These areas are good for photography from morning to around midday.
Because this is Seattle’s main airport the traffic variety is good. The dominant airline here by far is Alaska Airlines and its regional affiliate Horizon Air, followed by Delta and Southwest. You’ll also see most other major U.S. airlines at this airport. The Sea-Tac airport has a large number of foreign airlines operating on a daily basis both from Asia and Europe: ANA, Asiana (from South Korea using the Airbus A350), Korean Airlines (from South Korea using the Boeing 787-9), Icelandair (seasonal), Lufthansa (from Frankfurt using the Airbus A340), British Airways (from London using the Boeing 787), EVA Air (from Taiwan using the Boeing 777-300), and a significant amount of cargo using the Boeing 747. With such a good variety its a shame they didn’t build some sort of observation area for enthusiasts.