What is the one quality leaders must possess to be influential and why?

Dear colleagues, there is no absolutely one unequivocal undisputed answer to this question, but there are strong pointers to the most likely answers to the question. What I personally believe is one of the top answers is "sound competence". Some may tend to see "trust" as the best response, while others may see "integrity" to be the one. Charisma, communication excellence, honesty, caring, are other strong contenders, however, I feel competence trumps all. Imagine you are attending a major conference on global warming; and you wanted to attend because you are unsure about which way to go in your future beliefs; whether to deny there is global warming or to become convinced of its reality and become an activist yourself, so you are attending because you want to believe or disbelieve. Would you rather see a politician (eg, Al Gore), or an actor (eg, Decaprio), or an old scholar from MIT in geology and geophysics deliver the key note speech? Which one of the three would you pick? Of course the knowledgable scholar. Unless you are a die hard fan of Decaprio :-). It seems that solid competence (knowledge, skills, and experience) is the strongest quality in influencing others. I could be wrong.

There isn't one. 

There is no one quality required to do anything: everything requires a combination of qualities.