Your goal with adding such a sentence to the exam sheet is to make the students relax a bit. You do that be making them feel familiar with the situation, remind them that although the situation is somewhat artificial, this is the same material you covered in class, and that it is going to be all right.
To do that, you need to write something which the students will feel is a greeting from you. That is, your 'teacher persona', the impression you give your students.
So: What are the students' general impression of you? If you are an empathic teacher, worried about the well being of your students, 'All the best..' is probably good. But if you are generally perceived as a very strict lecturer, it would not be well received. If you are perceived as a bit nerdy, a pun relating to the subject could be good. If you have a phrase you often use in lectures, use that.