The upper Solduc in low water is some fun water when you run it for the first time. I would say that would rate high on my tough river list. I have never run the calawah but would sure like to give it a try ( after I talk to someone who has run it).
As to good rivers to learn on I would start with the NF Lewis and the Cowlitz. After that the Kalama is good. Just respect the force (of the water) and be carful. Any river can eat a boat.