Note: Things down the list have a lower priority; I'm not imagining I could grow all that on any useful scale on my own.
Everything more or less related to cabbage has plenty of vitamins, so it would take one of those.
The corn-bean-squash symbiosis that the natives traditionally grew has proven to grow well on this land AND cover basic nutritional needs. (also corn is the only land vegetable that has a decent amount of iodine)
Tomatoes. They aren't nutritionally essential, but they usually turn out quite plentiful and they're easy to preserve for winter.
Fruit trees, of the sturdy kind that don't need much care. And berries, for the same reason. You dump seeds in lands too rocky or difficult to use and forget about it. Won't feed you before years, but you lose nothing since they grow on their own.
Nastursum. These flowers are edible, but most important they blossom early in the summer and stay covered in flowers until winter; good for attracting bees. Bees are important for every other crop. The seeds are a decent pepper substitute too. Though if there are unused fields around, they should have enough wild flowers for every bee's tastes.
Everything else that will grow, for variety.
Something fermentable. I imagine that alcohol will be as good to trade as ever. And the yeast itself; people will also need to make bread, keeping a happy little yeast culture will benefit everybody.
And I'll say weed now! Actually hemp, but with the current state of things the smokable kind may be easier to find
But either will provide textile fibers, and we'll need that eventually. I hear cotton needs hotter climates.
If a forest area is nearby, I'd preserve it to have maple syrup, mushrooms and preys. (Canada doesn't want you to know this, but birch trees make a decent syrup too.)
