I would use plain tap water, but my local water plants uses chloramine instead of chlorine. Chloramine takes weeks to evaporate, if it does at all. I use ReptiSafe, which removes both chlorine and chloramine. It's possibly overkill, and chloramine may do nothing to reptiles, but I'd rather not take my chances. Before that I used bottled water, which I buy anyways for myself.
Plus by itself the tap water tastes nasty. Don't know if ReptiSafe makes it taste any better, haven't exactly tried it lmao.