Mutfruit are slightly larger than many of the other crops, but give +1 food, instead of the +0.5 food provided by all other crops. This makes them the most efficient for both space and number of settlers that need to be assigned.
In order to meet the food requirements for the maximum 21 settlers, three settlers can tend 18 Mutfruit for +18 food, leaving a fourth settler to be assigned to either 3 more Mutfruit, or 6 other crops.
Other crops might add variety, including the components required to make Vegetable Starch.
I'm not quite sure which crop takes up the least space, but if it comes to efficiency, Mutfruit is by far the most efficient. It is the only crop that yields 1 food per unit, as opposed to every other crop, which yield only 0.5 food. Mutfruit also doesn't take up particularly much space, though it might require slightly more space than Corn or Carrots.
Since Mutfruit produces twice the food per unit, it's also the most workforce efficient. As each settler can tend to at most 6 units of crop, a single settler can produce 6 food worth of Mutfruits instead of 3 food worth of any other crop.