Stevia.

I personally think it is DELICIOUS, though you have to be careful to get pure stevia and not the kind that's cut with other stuff.

Stay away from xylitol. That stuff is SO not natural, no matter what "they" say. The production of it typically uses GMO corn, too.

Brown rice syrup is good, too, but it's not really a sweeten-your-drink kinda sweetener. Just for baking and stuff.