I've tried a zillion recipes from other people touting the best-tasting smoothie. For some reason, I've always been disappointed. I even think a lot of the smoothies at fancy juice bars are lacking...and it's not because they're missing an important ingredient.
Rather, people just put too much into a smoothie. It dulls or overwhelms the bright flavors that should shine through when yogurt, ice, vitamin supplements, or other special add-ins or thickeners become part of the mix.
The best smoothie I have ever made--and I've experimented a lot--is simply frozen strawberries blended (in a blender) with fresh orange juice (not from concentrate). The tartnesss of the orange juice with the sweetness of the strawberries is perfect. No need for ice, since the strawberries are already frozen. And it's healthy, because it's fruit. Try it. You'll see.