I find carrying water to be one of the most difficult aspects of hiking. Its VERY heavy and the containers are often less than convenient/comfortable to carry. Even worse to hand carry or strap over a shoulder. Plus, placed in a pack/bag they bring a risk of leaking onto other gear.

I never found anything better than using 1 liter Lexan bottles, but always worry about them leaking, so I single or double bag them in gallon size ziplock bags, depending on what else is in the pack/bag. Never found a flexible container I trusted.