Variations on a single theme by way of an answer to this might be things like: 'the fundamental problem we face', 'the one that keeps us awake at night' or more simply 'one that's hard to answer'. And while each of these is a perfectly reasonable, factually accurate response, none is particularly helpful in the context of the issues faced by brands.
So, let's look at the question again, from a different stand-point.
I'd argue that what makes a hard question difficult, is not defined
from the perspective of answers (how many marketing solutions are
actually very simple). Instead, the hard part is being able first to
recognize the right question to ask and second in posing that question
in language that helps to stimulate an engaging and persuasive solution.
So here is an alternative definition based on this thought:
The Hard Question is the one, that amongst all the questions we could ask, is the one we should ask. The difficulty comes in developing the strategic insight to be able to identify the right question, and having the creative insight to ask it in the right way.
While this may sound deceptively simple in theory, it's rarely pursued in practice. When faced with a problem, our natural inclination is to jump straight to solutions. Answers are of course the ultimate objective, but the best way to get there is to invest time in getting the question right first.