I have tried several options but the best ones for me are the Camelbak products. I like the H.A.W.G. and the Transalp. They both allow for 100 oz. of liquid and ample storage space. The liquid is somewhat insulated so it stays cool for quite a while. They are not like the skimpy basic units that only carry liquid and maybe and map or something small. These bad boys can carry rain gear, food and some essential equipment. Currently Popular Surplus here in AZ has the Transalp on sale, usually $129.00 now only $79.00. My two sons have the H.A.W.G. and really like them, but they are just a bit smaller cargo capacity, but a very nice balance. I use the Transalp. Check them out.
--Sam