Studies show that tap water supplies in developed countries are generally a lot cleaner than bottled water.
Water bottled in plastic should not be a regular drinking source for anyone, not just because of the plastic contaminants that leech from the bottle into the water, but also because the pollution caused by every plastic bottle you throw away, contributes to the very pollution you are trying to avoid when you search for healthy sources of water.
Water bottled in glass is generally a lot better, with studies showing that the same brand of water from the same source, contains far fewer plastic contaminants when bottled in glass, than when bottled in plastic. However, glass bottled water still contains contaminants from the source water, the bottling process, and from the plastic inserts in their lids.
Tap water is far more closely regulated for contaminants in most developed countries than is bottled water, with utility companies being required to submit annual reports of contaminant levels in tap water, and no such requirement existing for bottled water.
I couldn't find many studies directly comparing contaminant levels in tap water vs. glass bottled water. The best I could find was a 2022 review study, where researchers found that glass bottled water had higher microplastic contamination than tap water. They theorised that the microplastics in glass bottled water could come from the source water, from the bottling process, and from the bottle lid. They attributed the low microplastic contamination of tap water from a high removal rate of microplastics in tap water treatment plants. Plastic bottled water, by the way, was the worst by far.
Despite the findings of the above study, there's still a chance that a particular brand of glass bottled water is cleaner than your tap water, but the only way to know is by having them both tested. However, since glass bottled water isn't a clear-cut winner against tap water, and bottled water in general is some 1,900 times the cost of tap water, with glass bottled water being even more expensive, include on top of that, the fact that both plastic and glass bottled water is far more resource intensive and polluting for the environment than is tap water, I prefer to just stick to tap water.
The best thing about tap water, is that since it's so cheap, we can add a slight expense to it, by using a filter, or distilling tap water to make it even cleaner.
So to answer the question, what's the best water to drink?, I'd say it was filtered or distilled tap water.
There are several different ways to filter your tap water, you can have in-line filters installed under your sink, a reverse osmosis system, or a water filter jug (“pitcher” if you're in the U.S.). Most people get started with a water jug, since they are cheap to get started with, and don't require any plumbing or installation.
But not all water filter jugs are the same!
Most do little more than improve the taste/smell of your water, they aren't certified to remove any health contaminants like heavy metals, disinfection byproducts, pharmaceuticals, or pesticides. Some even add dangerous contaminants like lead to your water. Many popular brands are made in China!
You can find out what I believe are the best (and worst) water filter jugs here.