They tend to be the ones I just finished a job for or just got paid for
Right now I am very fond of Mintel, for whom I did a batch of shops last week that are likely to be paid this week and Trendsource, where I picked up some Business Verifications today. But then there are the guys who have restaurants and the guys who end up bonusing nicely in my area and the guys I picked up a dozen grocery stores from today for July. Over time you will find the companies that have shops you like in your area. For the most part my favorites are eclectic enough because they deal with clients local to my area as opposed to serving giant nationwide chains. I have identified about 30 companies out of the more than 200 I am signed up with who 1) have shops in my area, 2) are reasonable to work with and 3) pay in an acceptable time period. I am signed up with every company I can find because every year a few of my usual companies lose their local clients while other companies add clients in my area.