Firstly, the A-Level specs for all of the exam boards hardly differ from each other.
Secondly, most students following Edexcel have told me that they consider MEI to have been the hardest exam papers; but most of the MEI students have said Edexcel papers have proved to be the most difficult.
Thirdly, I'm really not sure how you might objectively determine that a particular tricky A-Level question is more difficult than another one.
Finally, my own measure of the difficulty of a paper is in terms of how long it takes to complete. eg how many minutes needed per mark available. (or marks per minute, of course!). And then compare to Paper spec average marks per min.
In that way, all of the exam boards have remarkably increased their difficulty over the last 4 years. 18 of my top 20 tricky A-Level questions (pure) have come from the last 3 years, with a fairly even mix of exam boards.