One thing that contains everything the human body needs: a healthy human body. I'm pretty sure that's not the answer you are looking for, but it is a proven solution of quite a few primitive groups with malnutrition problems.
But really, if I had to choose one thing to eat forever, my answer would be animal, preferably ruminant, eaten in as great a variety as possible (not just muscle meat). So a mix of all the ograns, stock made from the bones, fat, and meat. I like Kurt Harris's view that there are no essential plant foods and that it wouldn't have made sense for humans to need a ceratain plant in the diet to thrive. That being said, I think vegetables are a great addition to the diet and that they foster better health.
Kurt Harris post: Archevore - Archevore Blog - Plants and plant compounds are not essential or*magic
Based on a post by Mark, here is the menu:
Spinach and Hard Boiled Egg Salad
Wild Alaskan Sockeye Salmon with Extra Thick Skin
Bone-In Grass-Fed Cowboy Ribeye with Grass Fed Butter Melted on Top
Okinawan Sweet Potatoes with Grass Fed Butter
Blackberries, Grass-Fed Greek Yogurt and Macadamia Nuts
Ancestral Health Info
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Primal Blueprint Explorer My blog for people who are not into the Grok thing. Since starting the blog, I have moved close to being Archevore instead of Primal. But Mark's Daily Apple is still the best source of information about living an ancestral lifestyle.