Another vote for Shuttlepark 2 -- their shuttles seem to come around less often than Masterpark, SeaTacPark or Doug Fox, but Shuttlepark 2 is all park-and-lock and usually cheaper than most, especially if you print out the current coupon on their website.

One of the enduring mysteries of Sea-Tac parking is whatever became of Shuttlepark 1.

Another good option is the newish, fully automated (except for the shuttle drivers) MPark.