Personally I recommend good old bottled spring water, with the minerals God intended it to have. Reverse osmosis can also be used to purify spring water, however. What I don't understand is people buying bottled purified tap water -- why not just buy a purifier for your sink and be done with it? The only reason to buy bottled water is to get true spring water, purified naturally by centuries of coursing through underground aquifers. What I don't like about reverse osmosis is that it takes all the taste out of water. Good spring water tastes like water, different springs have different tastes. Tap water tastes awful because of the chlorine, and purified tap water has very little taste at all. Reverse osmosis even takes the taste out of spring water. There are other purification methods that don't do that, which again is what I personally prefer.
RO water hydrates, but has way less minerals. Not much iron. Depending on how advanced the RO system, its actually possible to take all the minnerals out. So thats why they add minerals back in at determined values.
Interseting fact: 100% pure H2O will not even conduct electricity! Its the mineral content (metal and salt) in water that lets electron flow happen. Please dont try this at home. This is used in industrial electronics cleaning now that you cant use freon.