Most nutritious and yummiest? Whatever is local, in season, and you feel like. Ripe bananas make a pretty good base with berries or whatever else. When I was in Washington in the winter, I did a lot of apples, then added greens (spinach, kale, chard, romaine) and some spice (ginger, cinnamon, clove, cayenne). Summer I did lots of banana and blueberries. Also frozen bananas with some kind of nut like almonds or walnuts makes for a great ice cream!! :p Toss in some chocolate and mint and you've got heaven.
Just do a search on here and I'm sure you'll get a bizzillion recipes . . .
I don't think this exists for humans yet, unfortunately - but what I think would be the healthiest mattress (and virtually free of dust-mites, even bed-bugs which inflict some homes recently) for humans would be the same as the healthiest for pets...