In reply to Jamie Bankhead:
I think its difficult to say what the single hardest problem is as there are so many hard problems that have seen so little or no attempts/repeats, that grades cannot be confirmed. For example there are the numerous unrepeated Gaskins 8Cs, Paul Robinsons Lucid Dreaming 8C, Fred Nicole's unrepeated Le Boa 8C, Ondra's unrepeated Terranova 8C+ etc.. all of which are definitely nails hard, but haven't seen sufficient attempts/repeats to call them the hardest. Another prime example would be Fred Nicole's Oliphant's Dawn in the Rocklands (which he never graded just suggested around 8B - 8C), which has seen attempts by ondra, and supposedly daniel woods & paul robinson and so on, all of whom couldn't touch it, ondra even quoted 'after one hour of sessioning I walked away having done none of the moves. And I heard that bunch of people walked away with same result. All my esteem to you, Fred!', so must be pretty damn hard!
The only ones I can think of that could be confirmed to be up there as the hardest (I'm sure there are loads of others) would be Christian Cores Gioia, which as mentioned above Ondra suggested an upgrade to 8C+ and has also seen failed attempts by daniel woods amongst others who all suggested it was extremely difficult. Also Nalle's Livin Large 8C in the rocklands has yet to see a repeat and again had been tried by loads of very strong climbers, including Ondra.