The most ideal beverage, with zero calories, and 100% hydration is Water! 

Make sure to drink 6-8 cups of water each day. If plain water is not your thing, why not spice it up a little with a few slices of lemon, lime, or cucumber? Refrain from flavored water.  Almost always this flavored water can be very high in sugar and artificial sweeteners.

Always check the nutritional label.