I would have to say Distilled water, because there is no chlorine or other chemicals that are added for "taste" like there are in the regular tap.

You probably don't want to to use "mineral" water either...for reasons that Omega said...you don't know which concentration of "minerals" are added to the water. You could probably go with "purified" water, since that is basically the same thing as distilled.

I have only been using distilled water, and it's been giving me no issues at all

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