Cold, fresh water, brought to temperature in a kettle.
Why cold water? Hot water has passed through your hot water heater, which may add mineral deposits to the water which make the water taste funky. (By the way, the same goes for coffee.)
Why fresh water? Heavily filtered water, or water that has gone through reverse osmosis without adequate re-mineralization, doesn't have the fundamental building blocks of good-tasting water, and may actually be less able to extract flavorants from tea leaves. (Again, the same goes for coffee.)
Why not microwaved? First of all, many varieties of tea are not supposed to be made with boiled water, but rather with water just below the boil. Oolong = 190F; Green Tea = 155F; etc. It's very difficult to tell what the temperature of the water is in a microwave. Also, microwaving water tends to leave the water in place, while a kettle tends to heat water from the bottom up, and the water rises when heated, thereby agitating the water to ensure even heating.