Some people say, do the easiest thing first. After all, it is done and over with quickly and you feel like you have accomplished something. I on the other hand say, do the thing you dread the most. A. because it is Done and B. you really HAVE accomplished something!
Both ways will give you momentum to move forward. But by doing the task you dread most you also have the added benefit of no longer feeling anxiety over it. That's a lot of extra baggage you don't have to carry around any more. Phew! what a relief.
And if you are dreading because it is such a big task. Break down into smaller accomplishments (but still do them first) ... just my 2¢