In ancient times, the tea scholar Lu Yu said the best water for brewing tea came from the center of a swiftly moving mountain stream. Given that few of us have access to clean, fresh mountain water for tea today, you may want to consider other options. Here are the main types of water and the advantages/disadvantages to using each type of water for tea brewing.
With any pH above 7, it's best to filter before brewing to prevent the unwanted flavor of extra minerals.
Well water poses a special problem for tea brewing because its pH is almost always above 7. Test your well water. If it's above about 8.5, it is hard water and it will brew into bitter tea. Be sure to filter it.
If you bring it to just shy of a boil after filtering and you can still smell minerals, it's still too hard for tea. Consider using bottled water instead.
If you have bad tap water, you may want to consider using bottled water to brew tea. Mineral water is too hard (mineral-rich) and may leave your tea tasting metallic or harsh. Distilled water is too soft (low in minerals) and will brew into flat-tasting tea.
Quality spring water is the optimal bottled water for tea, but some types are better than others. The best spring water for tea should be neutral in pH (about 7) and in flavor. It should have a Total Dissolved Solids (TDS) content of 30 parts per million (PPM) or less.
If you're deciding between spring water and good, filtered tap water, you may also want to consider the cost and environmental impact of using bottled water. If your filtered tap water is nearly as good as the bottled water, I recommend using the tap water.
Aside from fresh mountain stream water and good-quality bottled spring water, filtered tap water is generally the best option for brewing tea.
Some neutral-tasting tap water may not even need to be filtered.
Hard water should always be filtered for the best tasting tea. However, if you bring it to just shy of a boil and you can still smell minerals, it's still too hard for tea. Consider using bottled water instead.
I use a regular Brita filter, but there are more sophisticated water filters on the market for those with specific water problems or a desire for truly excellent water. Another good filtration option is Japanese bamboo charcoal, which is basically a simplified carbon filter. If you are using a filter, it's best to brew with freshly filtered water. (Water absorbs odors over time and, if your water has been sitting out in your kitchen for a while, you tea's flavor may be negatively impacted.) For more information on water filters, here's an article on types of water filters and a listing of top pitcher-style water filters.
As soft water produces flat-tasting tea, some serious tea aficionados warm soft water with certain types of rocks in the kettle. This adds a better balance of minerals to the water and the final brew.