I've had good luck shopping for and getting paid by Bare, Consumer Impressions, Corporate Research, Customer Impact, Customer Perspectives, ICCDS, Informa, Maritz, Market Force, MSI (Trendsource), New Image, QAMs, Satisfaction, Second to None, Bestmark, Jancyn, Feedback Plus, Person to Person, Sentry, Intellishop, Strategic Reflections, Consumer Service Experts, Sinclair and Business Resources. Hope I didn't leave anybody out; if I did, it wasn't intentional. No particular order here, all good.
There are many, many other reputable companies, and I've signed up with lots of them but have not yet been able to pick up shops.
I've only had a couple of issues. I did ten big box audits between Feb 2010 and Oct 2010 for GFK, who paid promptly. I wasn't picking up audits from them anymore, but they cut me as a shopper in July 2011 after I made a critical post responding to another poster. I also signed up for two shops with DSG. When I made the required call before the first shop, the conversation didn't go well so we didn't go forward with our relationship. I'm sure lots of people work for both these companies without a problem.
My definition of the best company: The one that pays me.
Mary Davis Nowell. Based close to Fort Worth. Shopping Interstate 20 east and west, Interstate 35 north and south.