Rainwater

Rainwater is pure water, nothing dissolved in it. No hardness minerals, no nitrate, no carbonate (KH).

I say it's pure water but in areas where there is air pollution, or near agricultural areas which regularly spray crops, contaminants do dissolve in the rain as it falls. As do bird droppings on a roof where the rain is collected. If there are none of these, rainwater is good to use.