I just used tap water, although I did install a whole-home water filter about a year ago, so all the water goes through a pre-filter, a pretty substantial filter with carbon filtration, and then also a water treatment process called TAC (Template Assisted Crystallization) which doesn't get rid of hard water but makes it easier to clean and remove hard water scales.
I pretty much just make Americanos these days at home, but I have yet to have a better cup from a store and I don't really buy expensive beans or anything.