I like Nalgene .. it's light, tough, and the water still tastes good after it's been in the bottle a long time.

Most water bags have a tendency to break - unless you can stow them somewhere that protects them from impact.

Metal water bottles migh be OK - but they're too heavy for me if I'm carrying a backpack.

In an pinch - always remember that your basic large-size plastic soda bottle will hold a LOT of water. The plastic they use to make those bottles is incredibly tough and tear-resistant. Of course, if possible wash it out first - so you can remove the after-taste of the soda. And I don't like the narrow neck of those soda bottles. But Hey, let's not be picky if we're talkin' a real emergency here ... then those bottles are tough, hold a lot of water, and get the job done.

Pete