Depends on the kind of person you are.
In Biology (OCR), we learnt Biological Molecules first as it was apparently the hardest, but I found Cells pretty tricky because of the level of detail you had to remember. Also, plant transpiration and things was difficult because it was so damn boring.
And I found all of Physics (OCR) hard, but all of Unit two was quite difficult as we had to learn it so damn fast.
But like I said, it depends on who you are and where your weaknesses lie.