Well, both Fermat's last theorem  and the Poincare conjecture have been solved, so that'd make the Riemann hypothesis the most difficult by default.

i dunno(dont know not Duh No) but i still guess reimann hypothesis even if it was solved.
i mean there all easy problems its just humans cant handle em. but of course there all utterly impossible for almost every human even euler but dunno about gauss.
poncares got a 250 page solution about 100 bigger than that of Fermat.......
and WOW Xeda you've actually dabbled it ,attempted it YOURSELF?   but ...uh didn't you know it was sort of hard........and it would be groundbreaking.......
i cant even understood Boiincavae.
is it Poncarae or Boiincavae

i cant even understood Boiincavae.
is it Poncarae or Boiincavae

"Poincaré conjecture," even though it's not technically a conjecture at this point. It's actually very simple, if you don't get too far into the details of it. Basically, that the PC says is that if you take a closed surface* and put a loop around it, the object is topologically equivalent to a sphere if any arbitrary loop can be contracted to a single point on the surface. In other words, the surface can be deformed without "breaking it" in such a way that it becomes a sphere. This is a very rough conceptual image of what it says, but it's close enough for most purposes.

*essentially just if you take a sheet of something with no holes in it to see the inside, like a hollow sphere.