I have seen in many places (books, lectures, ...) that people say that some unsolved problem is more (or much more) difficult than another one or sometimes they point some problem as the most difficult one. How they know the difficulty of reaching an answer when they don't know the answer? Especially, as quoted by Andrew Wiles:
"You enter the first room of the mansion and it’s completely dark. You stumble around bumping into the furniture but gradually you learn where each piece of furniture is. Finally, after six months or so, you find the light switch, you turn it on, and suddenly it’s all illuminated. You can see exactly where you were. Then you move into the next room and spend another six months in the dark...
As an example of unsolved problems consider The Millennium Prize Problems.
And to mention a few of [the contradictory?] facts about it :
1- Clay Mathematics Institute had considered Poincaré Conjecture among the others but it is solved just after 2 years the list was published (or at most 7 years if we consider Perelman's starting time of focus on the problem).
2- Michael Atiyah states Yang–Mills existence and mass gap to be the most difficult one because it requires more fields of knowledge to work on, as a criterion. While hearing from people in maths when they come to say (or say metaphorically) the most difficult unsolved problem they usually point to Riemann hypothesis. I have also seen several written texts about that claim for example the quote by M. C. Gutzwiller :
"The zeta function is probably the most challenging and mysterious object of modern mathematics ..".
I don't think deciding which one is more difficult is some personal naive opinion because for example almost all the great mathematicians in history [esp. Euler, Gauss and Hilbert] have refused to spend long time on the Fermat's Last Theorem because they knew they won't be successful at it. But how they knew it? I would like to mention Hilbert's quote on Fermat's Last Theorem on why he didn't try to solve it : "Before beginning I should put in three years of intensive study, and I haven't that much time to squander on a probable failure." How he knew that before trying it for a long time?