What is the purest bottled water?

I'm curious, what exactly do you mean by the 'purest bottled water'? Is it a matter of the source of the water, the filtration process, or perhaps even the packaging used? With so many brands and types of bottled water on the market, how can one truly determine which is the purest? And furthermore, does the purity of the water even matter when it comes to drinking it? I'm eager to hear your thoughts on this matter.