There is no right or wrong smoothie combo. Think of yourself as an artist creating an artisan beverage to suit the needs of your body. Create a freezer full of frozen fruit options plus a pantry and fridge of vegetables. Each day you can paint a smoothie to suit your needs.
Buy what is in season or readily available at a good price. I am always looking for over ripe discounted bananas. The brown speckled ones have the best nutrient value and are super sweet too.
Your freezer will stop them from further ripening. Simply peel, pack and freeze for weeks of banana sweetness in your smoothies. See Smoothie 101 – Freeze Ripe Fruit! for more information.