Black Diamond Axiom 40 Backpack - 2440-2563cu in
BD is on the ball with their new reACTIV suspension system. It allows the pack to stay snug on your back but still allows free movement of the shoulders. This will reduce the amount of shoulder fatigue and won't make you feel like you have a piece of plywood strapped to your back.
The pack is super comfortable and the openair back panel keeps your back from getting all sweaty.
This pack also comes in a 30L. Item# BLD1172 and a women's version. Item# BLD1175