What I found after a long search. (Thank you for your valuable comments).
Softness/Sweetness of water is based upon the mineral content in it. Lesser the salt and mineral content in the water, sweeter/softer the water is. Commonly, water bodies close to a water fall have sweet water because deposition of soil in such water bodies is the least.
As we all know, no one can practically measure the taste of water from all the water bodies on earth, and compare them.
Now, although Siruvani reservoir is widely claimed to have the second sweetest water in the world. But I was not able to find any official source for this claim. And the Guinness Book of World Record for the 'Sweetest Tasting Water in the World' goes to Tomalistis Falls in the Caibiran municipality of Biliran, Philippines.
Thanks to Quora :)