How important is Master thesis when applying for a Ph.D.? I am willing to apply for a Ph.D next year and by September I am planning to graduate from a M.Sc. in Economics. I am trying to choose my mentor for my final thesis but I was wondering which is the impact of a Master thesis on a Ph.D. application.

I know there are many thing that will be taken into account when analyzing an application but I thought a good thesis in the field for which one is applying could be a nice introduction as well as the name of the professor (the more "famous", the better) chosen as tutor. 

Are my considerations right or wrong?
Sometimes choosing a famous name backfires.  If it's a hot shot with tons of students, he will distinguish among his students, when writing his letters, by giving strong letters to the strongest among his flock, and mediocre letters to the less spectacular.  Such a mediocre letter from a big name can be quite damaging to an application.

Even aside from that -- choose the person who fits best:


you are interested in their work
you admire the person ethically
they bring out the best in you
their management style suits you