This is a great question. There will be differences of opinions on this one, but in the end it is a very important question.
First it is good to define what an ecological footprint is. The ecological footprint is a the amount of the earth resources a person or nation uses. This includes the use of food, water, transportation, housing, waste management, and other things a person may need to sustain life.
In all my reading, it seems that the world's need for energy is creating the greatest ecological impact. If you think about it we use more energy now than every before. We need more and more energy and there is less of the earth's resources to meet these needs. Moreover, as the rest of the world wants to live like Americans, there will be more energy needs in the future.
The implications are far reaching. Think about what companies need to do to gain oil. Deep sea drilling is one of them. This is not only dangerous, but harmful to life. The BP oil spill is a case in point. To meet the demands of Japan's energy needs, they build nuclear reactors. Look at what happened this summer with the meltdown. In short, our need for energy is great and it is adversely affecting our environment. In light of this, the greatest impact on a person's ecological footprint is the need for energy.