My (partial) list:
Beyond Hello, Brandt Group, Jancyn -- respectful and pleasant communication on the part of schedulers and editors.
Market Force -- self-assign, multiple dates to choose from, sometimes with date ranges, self-reschedule, prompt payment, documentation (including CPI) that gives details on the exact specifications on the shop requirements, reasonable reporting requirements.
Corporate Research International -- for providing comprehensive, clear training materials. Yes, it can very annoying to have to complete training before even being able to see the rate for a shop, but I do appreciate the training being available. Those photos and detailed walkthroughs make completing that first shop a breeze.
Reality Check -- for the reason shopgal mentioned above: "editors that identify themselves by name when they are courteously asking for additional information or making a worthwhile suggestion for future reports". To expand on this, I like getting those constructive suggestions, in general. I want to know what I'm doing right, and where I can improve. A string of "9" ratings won't do me any good at all if I don't know what I could have done to earn a perfect "10". Editors who provide specific, constructive feedback help me to improve as a shopper and as a writer.
Intellishop -- for having schedulers who communicate promptly, effectively, and courteously, and are often available by phone. Also, for providing easy-to understand, generally clear guidelines, and for offering flexibility and a willingness to work with me when things don't go as planned. Their expectations are high, as they fully admit, but they have demonstrated respect for me, and are one of my preferred companies to work with.
There are others, but those are the ones that immediately come to mind as I scan down my list.