"What is your favorite insider tip in Paris?"
Breakdown:
* Quel: What
* est: is
* ton: your
* bon plan: insider tip (literally "good plan")
* préféré: favorite
* sur Paris: in Paris
This phrase is asking for a recommendation or suggestion for a good place to go or something to do in Paris that the person knows about. It implies that the person is looking for a hidden gem or something off the beaten path.