The most powerful word has always and forever will be
The way I look at it is for every idea, great or not great, there had to be someone that they told that told them "yes this is a good idea" "yes I will get behind it" or even them telling themselves "yes I want to go ahead and try this" This happens in every worldly instance, starting a war, getting married, you name it. Yes has to be it, with No in second.
"No" can be viewed as equally powerful since every one of those ideas or thoughts could have been halted with the inverse response. Imagine all of the ideas that never came to be because someone said "no"
It's gotta be a tie between "Yes" and "No".