The US Army Bladder and insulated cover has two quarts (equal to 1.9 litres) so this is ideal for long journeys, trekking and expedition use. The shoulder strap/belt clips make it very accessible without taking your rucksack off.
The internal bladder squashes down when packed away. Has a small external pocket for storing iodine/chlorine. As with all army issue equipment, it is made to last.