OK, I can see how rain water would be good. We have well water, and the plants love it. There's an in-line charcoal filter and water softener and I drink it and water plants with it. Daisyl is correct that distilled is bad. Otherwise when I've lived elsewhere, I've used tap because I paid for water service and why have water service if I don't use it, right? (except for known areas, which is still shocking). Most times, I would use a Brita filter pitcher for anything we would drink if we didn't have in line filters. If you use tap, have your water tested if you really want to get into it, then adjust something else you are doing for the plant. I should note that I do not try to keep my plants perfect, I try to keep them healthy overall, so my goal would be different than someone who, for example, shows orchids, etc.
Long story short: The water you use depends on your goal.
