I'm real partial to the Eastern half of the Alpine Lakes Wilderness. Pretty much anything E of the Cascade Crest and above 5000 feet is awful nice country! 
  
Having said that, it is not the most desolate place to go hiking. Popular spots can be a bit crowded on the weekends, though if you are willing to go off the trail there are some spectacular gems out there, and no people!  
  
The only part of the ALW that is permitted is the Enchantments, Lake Caroline, and Eightmile Lake if I am not mistaken. A NW forest pass is required for parking though.