Depends on where you are. For me, it's the water I get from my filter system. It leaves enough minerals in the water to be really good for brewing the styles I brew. Some brewers in the area use straight tap water. Others, in different parts of the country/world, use other waters. Some store bought, others from RO systems, some add things to their water to make it optimized for what they're going to brew. If you're going to add things to the water, you need to know EXACTLY what's already in it for best results (usually a lab report on a sample you send in).
I would start by at least getting the publicly available water report from your source. I believe that's available to anyone in the US, with a search on their town's web site. Or, at the very least, from the water company's web site. Some are more detailed than others.