It would be whichever plant develops the most biomass per unit of land in your area, fast-growing plants use more than slow-growing plants. Typically, this would be grain crops (wheat, rye, oats, corn, etc.); these require a lot of nitrogen to be healthy and productive. It could be a different crop in other parts of the world (bananas in the tropics for example). It also depends on the area the crop is growing in; corn for cool-season areas often make small plants, 4 or 5 feet tall maybe, relative to corn in prime corn-growing areas where corn might grow 10 to 12 feet tall. The taller corn uses more plant nutrients and water.
Some people might think that squash and related plants might qualify, but their root systems tend to cover a small area relative to the size of the plant, and the plants are usually widely-spaced. The soil those roots are growing in must be well-fertilized though.
Fortunately, whatever crop you are growing, the nutrients taken out of the soil can always be replaced. We can grow many crops that have high nutrient uptake in successive years as long as we are good stewards of the soil and put nutrients back in the soil (putting back at least as many nutrients as our crops have taken out).
