Often we do not have time to read all the questions and decide which is easier and which is tougher..
First if the examination paper consists of a number of subjects together, then we choose the order of the subjects from most comfortable to most difficult.
Generally the topics or chapters in a subject are easier at the beginning and become tougher at the end after 10 chapters or so. So we are more comfortable with the topics at the beginning.
So it is preferable to attempt from the first question onwards. Read the question, decide quickly whether you can solve that or not. Solve it quickly if you can, or at least write down most important points that are part of the answer. Move quickly to the next question and do the same.
If you are an expert at a particular topic, then it is best to attempt that first. Like I am good at grammar and the answers can be short and quick. I may not be so good at essays. Then I will attempt the grammar questions first.
Often we can read and understand questions which are very brief. It takes more time to read questions spanning many lines. In that case, one may like to proceed to very short questions first.
We have to use the mix of these ideas above to decide the order of answering questions.