Hello jabelufiroz and good morning. This is a tough question for me. I am focused on two words now = power and possess. My view and short take is if I possess the power it certainly can not be the greatest. If it were then I could not possess it. That does not mean the reciprocal of the power possesses me, only simply I could not possess it.
That leaves in my humble view the greatest power that "we" as Human Beings possess is a power we can possess is not the greatest power or of greater powers. I ponder "what ifs . . ." with the next bit of pondering. I agree with the three answers presented thus far by Seshogopalan, Tusitala Tom, and JThomp42 emphatically. The power of choice and the connection with the power to think offer much and and balance with stasis too.
Yet, for me, I ponder the power of desire. Desire has a bit of magic both within it and surrounding it. Desire is mysterious as in a nature of obscure and too is mystical as in transcending human toward divine. Desire for freedom may have both a probability and a possibility of greater than having freedom and a desire to exercise it how one decides.
Desire with liberty is for me a greater mystery once we preface it with of, for, by, and of course "and." There are other prefaces too. Does that mean desire has power or is a power?
Power is a variable from my understanding or it does have differences. Desire does as well. One may ponder the desire to write a children's book, the desire to write a poem, or simply the desire to communicate. Communication is what is desired, however it is not the answer or "why" there is a desire to communicate.
Some spend (sadly spending means no longer available) a lifetime seeking to communicate somehow and someway. Many are here at HubPages seeking communicating something a result of a desire. Questions can be asked like is desire the fuel or the cargo of the flight seeking communication?
Perspectives may be if desire is the fuel for communicating something then what are the avenues for the vehicle of communicating. There are many - writing, art, music, or any combination of those suggested by Howard Gardner as the ten common talents or gifts of intelligence endowed by each individual.
Another question is if desire to communicate, like a children's book is accomplished and achieved, well, what does happen to desire? Does it diminish, sit idle, less focused, grow yet is chaotic, turn down a different avenue, or does desire have a propensity of stasis?