Boundary Trail in the Pasayten Wilderness. 90 miles end to end (Iron Gate to the PCT) plus (I think) 30 miles south on the PCT to hike out. Requires very long car shuttle or drop-off/pickup between Harts Pass and Iron Gate TH on the eastern end, but well worth it. You could also schedule the western pickup in the Ross/Diablo Dam area. You will see very few people! Resupply could be a problem--would probably require hiring a packer. This is part of the Pacific Northwest Trail, and the best journal i've read includes that section of trail here (note that this section of his journal starts sooner and ends later):
You could of course do two one-week loops, thus neatly solving the resupply problem. One, an outstanding scenic classic in the Glacier Peak Wilderness, is the Spider Meadow/Lyman Lakes/Image Lake/Buck Creek Pass loop. The car shuttle between the Phelps Creek and Trinity trailheads, is only 2 miles; if you go in or come out on a weekend you should be able to get a ride. If you're going to walk it, do it at the start of the trip, not at the end; the first day to Spider Meadow is a short and easy 5 miles. A second loop would be the Chelan Summit Trail, in the Chelan/Sawtooth Wilderness, which can be done as a neat if somewhat strenuous loop using the Lady of the Lake ferry on Lake Chelan--drop off at Prince Creek, 4,000 feet elevation gain to the summit, hike the summit trail to Purple Pass, then a similar drop to Stehekin and return via the ferry (after visiting the bakery, of course!).