It seems likely that there is no board which is conspicuously harder or easier than the others, since firstly QCA, the body responsible for monitoring standards, would not allow it, and secondly, schools would all flock to the easier board(s). Heads of Maths would be quick to spot a board whose specification were much less demanding than the others; they are keen that their candidates achieve the highest grades.
I suspect that few contributors to this forum have sufficient experience of all the boards to enable them to answer this question, however!
Aitch