I second Cilogear 30L.
It is astounding how much stuff you can fit in it in addition to being very comfy on my back.
Nobody believes me when I say I used it on Rainier for 3 nights and 4 days.
Tent, rope, rack, half bag, thermorest, food and clothes - inside.
Crampons, tools, 1 picket and helmet - on the outside.
Here it is in action:
Big props to Graham!