Statistics is definitely the easiest... You hardly have to remember anything at all. It's all there on the formula sheet really!! And doing that, you might find lessons tedious but gives you plenty time to revise for your other exams!
I did D2 and it's a bit different to D1 but there are fewer, longer questions. But it takes many of the concepts of D1 such as linear programming and extends them into more variables. Like D1, you have to be so clear with your working you lose easy marks if you let the examiner guess what you've done, even if you get the right answer.
M1 is easy to those with the right mind for it. People who can visualise the problems and can "see" what's going on find it easier than those who can't. I found the actual maths of it quite easy, but it was setting up the model from the information in the question that was the difficulty for me. Once I actually worked out what on earth was going on, the rest just followed.