Well, that a good question! After visiting with many coffee people, I have gathered the following as a basis for recommending the "perfect water" for coffee.
1. All oxidants removed. (Chlorine or other such "sanitizers".)
2. All organics removed. (You know, dead fish, tadpoles, THM's, insecticides, pesticides, etc.)
3. TDS (total dissolved solids) from 60 to 100 ppm (parts per million)
4. Hardness of about 3-4 grains per gallon. (51.3 to 68.4 ppm)
5. Low sodium water, i.e., less than 10 mg/L.
6. pH depends on the Bean you are using, plus the method of extraction.
7. Iron, Manganese and copper gone, or less than 0.02 ppm.