How do you feel about valet versus park-and-lock?
If you're OK with valet, MasterPark seems to have frequent shuttles, and the lots are nice and close to the airport -- but I think they're expensive.
I prefer park-and-lock, so I usually use ShuttlePark2 (
www.shuttlepark2.com). 24-hour service, reasonable rates, you can make a reservation online in advance, and they award frequent-parker points. They're north of the airport too, so unless you're coming up from Federal Way, etc., it's an easy shot. The only thing is, it's been known to take a few minutes to get picked up on arrival.