I use rain water because it is the purest available to me, even more pure than the well water that is in the area. But it really depends on the location, I have been in parts of Tennessee and Pennsylvania where the water is amazing. Use animals for an unbiased test they can tell by smell what is the purest, I tried with my cats and dogs with 4 different water samples and they all chose the rain, animals have the instinct in them to know what is good for them when us dumb as humans will eat poison if its not labeled properly.