Organic Brown Sugar in my experience works best! Coconut sugar, Palm sugar, Coconut Blossom Sugar, Molasses, Organic white sugar, Sugar Cane Juice and Sugar Cane Stalks can be used as sugar source.
People choose to use Palm Sugar, Coconut Sugar and Coconut Blossom Sugar because of it’s Low Glycemic Index and because it’s not high in fructose and is rich in nutrient value. Sometimes the Grains get bored with the same sugar source and you can make a mixture of the sugars mentioned above. CAUTION: I’ve tried fermenting with Coconut sugar as main sugar source and it was a bit hard on my Grains at that time, it is best to use sparingly in a mix with other sugars.