I also use the 2 liter platys. Wonderful because they collapse as the water is consumed. Just about as light, and much cheaper, but bulky, are recycled pop bottles - best ones are 2 L and up. One of the very best, although I haven't seen them in stores lately, is a squarish one gallon Gatorade bottle. In the past we used rinsed Clorox bottles. They were useful because they had a comfortable handle. On several Grand Canyon hikes, we would carry full bottles in our hands down the trail a bit, then stashing them for the return trip.