I wear a Marmot AirFlow pack. pretty cool on the back.

Not waterproof, but that could probably be fixed with a can of ScotchGuard. Seems at least some of them come with a rain fly, which might help, mine didn't have one.

Pocket for hydration bladder on at least some models.

Super comfortable for me, nice adjustable sternum strap. I can't use the waist strap while cycling because of my short torso. Taller folks might be able to use it, however.