my vote is for Hannegan Pass to Ross Lake/HWY 20, going over Whatcom Pass and through the Little and Big Beaver Valleys. Probably 55-ish miles all said and done. The trail is solid, the campsites nice, and the route accesses some of the deepest territory in the North Cascades. As far as climbable peaks go, Ruth and Challenger are good options. did this traverse a couple years ago, took five days and really enjoyed it! highlights included tons and tons of berries, the view of Mt. Challenger from Whatcom Pass, and walking through ancient cedar groves in the Big Beaver Valley.