I have a big Camelbak as my primary BOB -- I've learned not to keep water in it, but I do have water to put into it if I need to walk and I have time to transfer it.

A smaller Camelbak is my go-to bag for any sort of hiking. It's got a large bladder but otherwise very limited room inside. Perfect for a PSK, a small FAK, some sunscreen and a snack. I have a couple of MOLLE pouches that get attached to it when I need a little more storage.

My smallest Camelbak is a lumbar pack that holds a little water and a few other things. I use it mostly for running and biking.