Some would argue that the most powerful word is "no" because it liberates you from tasks, thoughts and feelings that are negative or wasteful of your time/efforts.
But I'd submit that the most powerful word in the English language is "why." Yet I think that most consider using it as a sign of less power. That by admitting that they may not know the answer or that they have an answer but there might be a better one makes them somehow not the best at what they do.
Salespeople seem to be especially fond of this foible. Not sure why that is -- maybe because it takes such a strong ego to be good at sales -- but they just don't seem to be able to consider much less admit they may not really know their customer.
"Why" gives us permission to not just question but to consider. It opens up the exploration and discovery process and leads directly and exclusively to one place -- learning.
I hope more people start asking "why" more often -- especially in Marketing. I think it would make a lot of work much better and effective.