How much water do you think is best to have in a good, all-purpose primary backpack-based water blaster? The reason to specify backpack is that it is typically easier to carry additional weight on one's shoulders than it is on one's arms. That said, there's definitely a reasonable limit as well and each additional pound/kilo does add up. Pick your preference in the poll!
As well, for those interested, please explain why you chose the number range you did.
Sidenote: You can choose two choices here since some may like to have a smaller backpack for light-duty missions while having access to a larger backpack for long missions or battling in areas with limit refilling capabilities.