How important is Master thesis when applying for a Ph.D.?
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