This question is hard to answer because everyone who has Kefir, claims to have the best Kefir. If you have access to different Kefirs then the best thing is to find out which suits you best. Only you can say which is the best for you.
When you buy commercially grown Kefir grains, you are sure you will always get the same quality and balance of yeast and bacteria.
The Kefir grains are a biological production centre. Living foods like Kefir help to detoxify, support and balance digestion, and help in building up the immune system to counteract negative influences.
During the fermentation process the Kefir grains change normal milk into the Kefir beverage. Lactic acid, ethanol, acetic acid, carbon dioxide and other compounds are produced. Interesting as well, is that vitamins are produced during fermentation.
Yeast in the Kefir cultures have been reported as producing some vitamins. Kneifel and Meyer studied the vitamin production during Kefir fermentation with different milks. In cows milk it was found that there was an appreciable increase in pyridoxine (B6), cobalamin, folic acid and biotin. The concentrations of orotic acid, riboflavin (B1) and thiamine (B1) decreased. However, Oberman reported increases in the concentrations of thiamine (B1) and riboflavin (B1). It has been suggested that these differences may be due to the method of production. (Grassnik)