Welcome! Everyone had a different favorite becasue everyone shops for different reasons.
Marketforce is good for newbies, becasue they have easy shops. However, their pay is LOW. Once you have become confident with some of their shops, I suggest that you look at the bottom of the page to find a list of the 15 most discussed MS companies (MSCs) and sign up with all of those (except any that are most discussed for negative reasons, of course.).
Also, you will get a big head start by reading all of the topics "pinned" to the top of the list in the New Mystery Shoppers area of the forum.
Most of all, enjoy !!
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel
Poor customer service? Don't get mad; get video.