Of all survival equipment, your top priority should be prepper gear to supply clean water for drinking and cooking food. Two quarts of drinking water per day is considered the minimum, and three days without water and you will die a horrible death.
Under ideal conditions with a source of fresh, clean water you could just boil it for 30 minutes to kill any potentially lethal microorganisms and make the water safe. Or again assuming you have clear, fresh water to start with, you could also purify water chemically, or with ultraviolet light. But surviving doomsday will likely be much like war - unpredictable and chaotic - so counting on "ideal" conditions is pure folly.
It is likely you'll eventually have to settle on obtaining water from less than ideal sources (with dirt and/or other contaminates) where boiling just won't cut it. Assuming you at least have access to a fresh water source, the easiest, most cost-effective method of obtaining fresh drinking water at will is using a good survival water filter or purifier (like those below). Since we are preparing for emergencies, we shouldn't count on electric power being available. That's why we'll focus on human-powered (or gravity-powered) water filters.
But if you are like 80% of the people on earth, you also live with 60 miles of a nearly inexhaustable water source - the ocean. Making the overly salty sea water safe to drink isn't any more complicated than for fresh water, but you need slightly better survival gear. The Katadyn Survivor 35 Desalinator below is basically just a water filter, but it's pores are so small that it even filters out salt - making the water it produces safe to drink. Another alternative is using an emergency water distiller which can also deliver drinkable water from many sources including sea water!
Below you will find only the top prepper water filters or purifiers, that made the Hot-Prepper-Supplies.com 5 star review rating.