Maybe just out of ignorance, but my last choice has always been tap water. My first has been the Spring Water. I have API Tap Water Conditioner drops, but if I have to put chemicals in tap water so it's usable... something just seems "iffy" about that, so I just don't really trust tap water very much, maybe I should?
Considering that they charge you a dollar or two per gallon of water, tap water can be a very low cost alternative. I'd use that, unless I had a specific issue with my tap water. For example I have a friend that has to have a whole water treatment plant in his basement because the well water is so bad.
If can't use tap water, and you'll need more than a few gallons, consider getting you own RO unit to make pure water for you. If you go that route, you'll need to use GH and KH additives to get the GH and KH to where you want it.
Lots of advertising dollars go into convincing people that they need to buy bottled water as it is something special. But sometimes reading a bit on the water bottle will give you a better picture of what you are buying. Many times it is tap water that is "specially treated". That may only mean it is tap water that is put in special bottles!
There are no broad regulations on what is called Spring water. It may say it is pumped directly from the ground or someway indicate that it is from a spring but lots of public water IS pumped directly from the ground so they are not lying when they tell you that. They may just not be telling the whole truth.
The main reason most of us use a water conditioner is due to most public water supplies use chlorine or chloramine to help maintain a low level of bacteria. The chemicals used can burn the gills and other sensitve parts of the fish, so we treat it to make it safe. If you have well water or other water without chemicals, it is likely to be safe for the fish.