At the bottom of each page of the forum is our "Official List of Mystery Shopping Companies". The companies on that list are not scams.
It is impossible to tell you which are the "10 best" companies because different companies have different types of shops and clients in different parts of the country. Different shoppers qualify for different shops as well as different shoppers find different shops are worthwhile.
All of us go through the process of 'trying out' a number of companies to find out those that have work we like and that are worthwhile for us to work with. They constitute a different eventual list of 'best' for each shopper. Let me suggest you read through the forum discussions linked to on the "Official List of Mystery Shopping Companies" to see which ones make sense for you to register with first.