As a broad general rule, you want higher altitudes. They tend to be more remote with fewer people and livestock and relatively more "pristine" I think of places like the Gila Wilderness in New Mexico - there are many similar locations throughout the western US. One that comes to mind, although I have never hiked there, is the Trinity Alps in northern California.

There is a body of opinion that if you are drinking water that is coming from a high snowfield or spring, there is a good chance it is disease free. Also many suspect that many cases of giardia, etc. comes from improper sanitation (infrequent hand washing) rather than from the water itself.

You must weigh this against the official notifications that all uniformly recommend treatment of all water unless it comes from an official faucet which is checked biweekly for fecal coliform, in full conformity with official standards, blah, blah, blah. It is actually rather understandable that the authorities are going to be very cautious about recommending that you scarf up plants, critters, or water without treating for contamination.

Back when I was a young puppy and just learning about the outdoors, we sipped from many a spring, as long as it was relatively distant from habitation. I did this for a good many years, until the concern for giardia arose (late 60's; early 70's) as boomers started getting into the woods.

One thing that may have saved me is my habit of converting water from any source into that best of beverages - a nice cup of tea.

We are all on our own in foregoing water treatment. Like anything else, assess the situation, gather information, and decide.