Depends on your goals. If you are just looking for calories, you probably can't beat potatoes. You can grow 100lbs of potatoes in a 4ft x 4ft plot. Maybe even more than that.

If you're looking at maximizing nutritional value, sweet potatoes might be a good bet. You eat the leaves and the tubers. Similarly to potatoes, you can grow a large quantity in a small area.

If you're looking for luxury items that may be hard to find in a shortage, then you have a lot of options. Though they might take more or less skill depending on the crop and climate. (Like artichokes).