1. Raw organic veggies.
2. Cold-pressed fats, e.g. olive oil and coconut oil, and lots of raw nuts.
3. Grass-fed beef.
I'm not a big fan of dairy but I do give my DS Krill Oil daily (switched from CLO a few months back following Dr. Mercola's recommendations and I've seen amazing results, we used to get a few colds/ feverish flu every winter sometimes with persistent coughing and this winter he had only one day of a slightly runny nose!!!).
it's definitely hard to choose, because there are so many important foods, and you can't survive off of just three foods. properly prepared grains, pastured and grassfed meat, raw milk (pasturized but grassfed for cooking purposes), fermented clo/butter oil, liver, fermented veggies, fresh veggies, they are all so important.