For the very best flavor, we recommend using slightly alkaline spring water with a low TDS level (total dissolvable solids) <90 and pH of 8.5. Most bottled water lists TDS and pH on their labels.
Second, we recommend carbon filtered tap water that is freshly boiled.
Unfiltered tap water that has remained heated for long periods of time is the least ideal. Though this is still acceptable, if no better alternatives are available.