There is no official answer to which state has the cleanest water, because no federal agency publishes a state by state ranking of drinking water quality. Every list you find online is somebody's compilation, usually built from violation counts, and the thing it measures is not the thing you care about. What comes out of your tap is decided at a far smaller scale than a state.
What the published rankings actually measure
The lists that circulate are typically assembled from federal compliance data, counting how many water systems in a state reported violations of drinking water rules in a period, sometimes weighted by how many people those systems serve. That is a measure of reporting and enforcement, not of taste, hardness or the pipe running into your house. A state with rigorous monitoring can look worse than a state that tests less, and a single large system with problems can drag an entire state's score down while most residents are unaffected.
Water quality is local, not statewide
Two towns in the same state can draw from completely different sources, one a protected upland reservoir and the other a river downstream of agriculture, and be treated by utilities of very different size and budget. Groundwater chemistry changes over a few miles. Even within one town, the age of the distribution main and of your own service line changes what arrives at your tap. Averaging all of that into one number per state discards precisely the variation that determines whether you need treatment.
The document that actually answers your question
If you are on a public supply, the report you want is your own. The EPA notes that every community water system publishes an annual Consumer Confidence Report and maintains a finder to locate yours. It lists the source, the contaminants detected, the levels measured against the federal limits, and the disinfectant used. That single document tells you more about your water than any state ranking ever will, and it is free. Read it before you buy any equipment, because it usually names the one or two things worth treating.
Private wells sit outside every ranking
State rankings are built from public water system data, so if you drink from a well you are not represented in them at all. The EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, and that around 15 percent of the US population, over 43 million people in more than 23 million households, relies on them. A USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human health benchmark. For those households the only meaningful figure is their own lab report, and our well water testing guide covers building one.
What clean actually means to a household
People asking this question usually mean one of several different things: safety from regulated contaminants, absence of taste and odor, softness, or the absence of staining. Those come apart completely. Very soft water from a granite region tastes clean and is gentle on appliances but can be corrosive to plumbing. Very hard water from a limestone aquifer is safe to drink and destroys water heaters. Decide which of those you are judging by, then measure your own supply against it rather than looking for a state to move to.
Questions people ask about which state has the cleanest water
Does the EPA rank states by drinking water quality?
No. Federal compliance data exists for public water systems, but no agency publishes an official ranking of states by tap water quality.
Why do online lists of the cleanest water states disagree?
Because each compiles different data with different weightings, usually from violation counts, which measure reporting and enforcement rather than what reaches your tap.
How can I find out how clean my own tap water is?
Read your utility's annual Consumer Confidence Report, which lists the source, the contaminants detected and the levels measured, and is located through the EPA's finder.
Do state rankings cover private wells?
No. Rankings are built from public water system data, so well households are absent from them and have to rely on their own laboratory testing.