Typically hard water as its higher mineral content is more agreeable to yeast than "pure" water. Distilled water is likely the worst unless minerals or mash are added. Either way removal of chlorine is a must and can be accomplished by an open lid "breather" simply allowing the water to breathe open air for a day, removal of chloramine is very difficult and if your metro water system has chloramine you should probably use distilled or spring water.
Avoid sulfurous water "skunk water" as some of the compounds the yeast or bacterias can generate could distill into some very vile compounds. --DGary 15:19, 13 February 2007 (PST)
Chlorine and Chloramine
In recent years, more water authorities have started to treat water with ammonia in addition to chlorine. This treatment results in the formation of chemicals called chloramines, which are similar to chlorine in that they kill bacteria and aquarium fish and ruin beer. There are several methods to removing Chloramines, some easier than others. See MoreBeer: Removing Chloramine or [ http://beer-geeking.blogspot.com/2011/05/water-chlorine-chloramine-and.html Chlorine/Chlorimine] Beer Geek]for detailed instructions.
Issues arise as yeast eat chlorine and chloramine and produce 'chlorophenols'. These phenols have extremely low taste thresholds; you can taste them even if they exist in only a few parts per billion. Your beer will have a difficult to define 'sharp' or chemical flavor, and usually a plastic or band-aid aftertaste, like you just stuck your nose into a first-aid kit. If the amount is low enough, you might only be able to taste it when you burp.
Some styles of washes are helped by specific water profiles. These profiles may accentuate certain flavor constituents to help mimic a certain style or brand of spirit.