Getting a good result on your Masters or PhD work is essential for your further career.
Some tips for choosing the best supervisor.

1. Choose somebody that is very passionate about the field, and also have in depth knowledge of what you are going to write about.
2. Avoid people that have abstracted themselves away from the hands on work and become an abstract thinker.
3. Choose somebody that has been successful, that students and co-workers admire and take seriously.
4. Choose somebody that is on the way up, but still has a lot of experience from being a supervisor. A PhD student has too little experience for mentoring a Master student in a thesis. A Professor may not have the finger on the pulse of what is going on, especially if he is diverting into administrative tasks. Perhaps an associate professor is the middle way.
5. Be critical to the problem that is going to be discussed in the thesis. Will you be able to solve it and make any contribution to the field, or is in an unsolvable problem.
6. How is the work situation going to be? Find something that suits you, for example some wants weekly meetings, some will leave it all up to you.
7. What kind of students have this supervisor had previously, how were the results, and are you the same kind of type as the previous students.