No cuboids are particularly difficult to solve once you understand their structure and where they come in the cuboid classification. This classification was discussed by SuperAntonioVivaldi on his YouTube Channel, here
is the first in his series.
I also discussed this very thing with his permission on my blog:http://www.puzzlemad.co.uk/2013/01/first-there-was-cube-and-then.html
The hardest cuboids really are those that would have the most phases to their solution strategy and the potential for the most parity problems. In general the "most difficult cuboid" would therefore be one of the "Ultimate shape-shifters
" group - that is it takes the form: N x (N + E1) x (N + E2) where E1 & E2 are 2 different even numbers.
Until Calvin decides to mass produce the 2x4x6 designed and first made by Hunter these can only be found as bespoke mods or as Shapeways prints!
The first to be produced in this group was Puzzle Maestro's 3x5x7. I have it and it is magnificent
Obviously bigger becomes more arduous (although not really more difficult) and there have been fantastic strides recently in these puzzles.
Traiphum made the 5x7x9 (using CrazyBadCuber's design - thanks Dan) and it is fantastic too.
BUT to add to the difficulty you need to add something new - making an even order version will add the even order parity to the puzzle and Jeremy Eisenberg (Jerm on TP) has designed 2 amazing versions of which I only have the smallest (So far
). These are the 4x6x8 and 6x8x10!!!!
So if you really want the "Ultimate" most difficult cuboid then I would buy the 6x8x10 from Jerm's shop here
Or for a little less money then get the 4x6x8 here