Welcome to the forum Susanne!
Kudos to you for having signed up with so many companies already.
As to your question, most often you'll receive responses telling you not to ask that question. It's a reasonable question, but because this is a competitive, free-market industry, most folks don't want to share their "favorites" list.
Let me encourage you to read the articles in the "New Mystery Shoppers" section, especially those by Flash.
Also, if you'll search the forum for "list of favorite companies", you'll find some good lists, but you'll have to look back in time quite a bit. Most of the recent discussions on the topic have not been very helpful.
I'm sure you've already found the big players, such as MarketForce, Intelli-Shop, and Corporate Research International.
There's a link at the bottom of the page titled, "Official List of Mystery Shopping Companies." It's very helpful. Don't be scared off by negative reviews as every company has them. However, if you see an overwhelming trend of people complaining about not getting paid, you might steer clear.
I hope that helps. I look forward to reading your contributions to the forum.
AndrewTX
Certifiable