What is the best real estate agency for property in Amsterdam, Netherlands?

Depends what you are looking for. Houses? First look at http://www.funda.nl a Dutch site with almost all houses that are for sale.
Then a real estate agent. DTZ Zadelhoff I think is the most famous for business property. Cushman & wakefield is an english one that is active in Amsterdam.
I heard Ter Linden and Heijer is a good real estate agent for housing. It is member of the NVM which garanties a minimum quality. But there are several. Choose a NVM one I would say.