My favorites are any for which the work:pay ratio is acceptable; that not only consists of the fee, but also the conditions of business. Are the MSC's contactable, are problems easily resolved and do they use a single page report devoid of repetition? With respect to those companies with whom I currently work, I have completed almost 600 jobs for Strategic Reflections and 236 shops for Service Sleuth.
I am familiar with all three of the MSC's you listed and, in the distant past, worked with them, but two fired me and the third is rarely willing to pay my requirements.
On an unrelated note: Today, I received a notice of a shop at a hotel. The fee was $20, with a reimbursement of $100 for a total of four drinks and two dinners. While that may sound generous, predicated upon the location, it is a small amount of pay for a mammoth mound of work. For someone, though, it could be viewed as a treasure trove.