Platypus Water Bottles. These aren't cheap, but if you shop around you can get them for less than the prices you can find on Amazon. They are available in various capacities and with different tops e.g. a straight screw on, or one with a pull up drinker.
So why would you buy one, rather than just use a recycled bottled water container? Their main usp is that they occupy almost no space when empty. They are also very robust. My oldest one is about 5 years old and still going strong. If you use a hydration drink tube inside a rucksack they are compatible with all the common systems.
If I go camping I need about three litres of water for an overnight stop to rehydrate meals and drink. I try and get water as close as possible to the place I camp. If I used rigid containers they would be wasting 3 litres of space inside my rucksack for most of the day.
When not in use, I roll them up and store in the freezer to stop fungal growth.