For single day snow/ice climbs and long weekend BP trips, I like the Marmot 40XT. It weighs just a bit over 3lbs, has a crampon pouch, tool attach points, loads from the top, and is made of tough materials.
For weeklong trips, I prefer the GoLite Quest. It weighs about 3.5 lbs and has about 72 litres max capacity. For multi week expeditions I use my Arcteryx Bora 95. At a shade over 7lbs, it's on the heavy side but it's comfortable and can carry big loads.