Osprey Exos Pro 55 -- Nearly a pound lighter than the Exos 58 and featuring a more fine-tuned adjustment system, the 55-liter Exos Pro is the most comfortable ultralight thru-hike backpack in its weight class. A highly-ventilated AirSpeed backpanel supports light loads with ease while maintaining maximum airflow across your back, making it an ideal pack for your most long-distance endeavors on the Appalachian Trail, Pacific Crest Trail and beyond.