Good Morning
I am going to apply for a LLM at Nijmegen Univ, Groningen Univ, and Aix Marseille III Univ.
Which one is the most prestigious University?

You should check other board posts on here about Dutch Universities. As for Aix Marseille, I heard that it has a good reputation in France. However, if you want to go to the best schools, I would suggest looking at Paris 1 (Sorbonne-Pantheon), Paris 2 (Pantheon-Assas) and some other schools like Lyon, Nancy ...

As a french student I'd say that Aix reputation is miles higher than the one of Nancy which is a quite average law school. As for Lyon you could look to Lyon III, Lyon II have quite a average reputation as well.