The main reason you'll be looking to purchase one of the best hydration packs is to carry enough water to prevent you from dehydrating out on the trail. The capacity will vary from about a liter for hip packs and bum bags to three liters for a large trail pack.
How much you'll need to bring will vary depending on the length of your ride, the temperature and if there is somewhere to top up – the usual recommendation is about half a liter of fluid per hour.