Bases to me are nothing more than a liquid to start your smoothie with, other wise it is just a juice
To keep your smoothie simple you can just use water and or ice.
A lot of smoothie recipes call for milk as the base of the smoothie. Again trying to stay as healthy as we can and to avoid some of the problems of dairy you can use soy milk instead. Our household likes to avoid soy as much as we can as well so we use Rice milk and thats why you see it in the recipes here. Another great choice would be almond milk.
Coconut Water, I have recently discovered makes a great base for smoothies. It is fat-free, cholesterol-free, low-calorie, naturally rich in electrolytes, and great source of potassium. Personally Im not fond of its taste but mixed in a breakfast smoothie it is great!