Tap water - you never knows where that comes from - at least in the urban areas. Nor do you know if it contains additives like, chlorine, fluiride, anti-bacterial substances etc.
I'd say that the best still is a true well - if you can find any of those still in you country - where the water has been purified and sieved throught the ground for many, many years.
There are as well many purification techniques however, which cleanses from most contamination and attitives.
Personally I think that the techniques utilized by the antroposophes are very, very interesting in this respect. Simple. Not technology driven. Patience oriented, natural, beautiful even...Take a look at these three samples:
http://www.youtube.com/watch?v=MWPlpdVLil4
http://www.youtube.com/watch?v=I__btPn1qag
http://www.wikihow.com/Purify-Water
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