I only ever use the bottles that I buy water in from the supermarket. They're effectively free, you can crush them flat and they still won't leak when you fill them, they're disposable, they're light (about 40 grammes for a two litre bottle), they'll take high pressures so you can sit on them, they have screw caps so they're reclosable, you can fill one with warm (not hot!) water so you have a hot water bottle, you can get a spare cap and pierce a hole in it to make a fire extinguisher (or, if you're a Jeep owner, a brake disc cooler
), you can make a funnel and a cup with one just by cutting it in two and you can use one to start a fire.