Okay, just one man's view here, and I recognize that I haven't ridden all the possible climbs in the state - particularly those in northeastern Washington ... and nothing off-road, but anyway here's a quick look at a bunch of the major paved road climbs in the state. This is a simple view ... ranks the climbs based on their elevation gain and on the average grade of the climb - a 50-50 weighting.
So the toughest five climbs ... without taking weather into account or the maximum grade or whatever ... are:
1. Hurricane Ridge 5,200 feet / 5.3 % grade
2. Washington Pass from Newhalem 6,500 feet / 3.9 % grade
3. Johnston Ridge Observatory from Toutle 5,600 feet / 3.4 % grade
4. Mt Baker from Glacier 4,500 feet / 3.6 % grade
5. Loup Loup Pass from Malott 3,400 feet / 3.8 % grade
Here's the link to the spreadsheet with the data.
There is measurement system error in this - I used Bike Route Toaster for the elevation info and I adjusted the grade calculation for a couple of the rides that had significant descents embedded in the climb to reflect the climbing over an estimate of the distance net of the distance spent descending.