Ideally*, without considering convenience, water should be from a spring, and the spring should be located as far from civilization as possible.
The water should not be heated or micro filtered (meaning it's ok to filter it for large particles but NOT to filter out minerals, ie reverse osmosis)
Reverse Osmosis and distilled water take all the minerals out of the water so that while it's inside your body it is able to leech minerals from your system as it passes through. Heated water is not bioavailable to the same degree structured, untampered spring water is.
The water should not be treated with any chemicals, nor should it touch plastic if possible.
Second bests are aquifers, clean wells and isolated, clean lakes. Rivers would be ok too except that there are very few that aren't contaminated by civilization.
Your best source of moisture is raw food sources, but spring water is a close second.
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Obviously all of the above is ideal, if not convenient, and it's quite difficult in the modern day to fulfill all of these requisites.
Avoid municipal water like the plague. Don't buy bottled water, wayyyy too much plastic being used and for other reasons. Try to keep your water in glass whenever possible, you will likely notice the taste difference as well.