Obviously dining shops that offer pay + reimbursement are desirable, but I'm fine with reimbursement-only shops as long as they are restaurants I enjoy.
Texas Roadhouse shops (haven't seen any of those in awhile) are easy, and that is one of the few places I mystery shop where I will eat on my own dime. Well worth the amount of reimbursement given the quality of the food and the nature of the report. Freddy's is another place I will gladly visit on my own, so a reimbursement-only shop with them is fine by me.
I try to get any Habit lunchtime shops I can, but they go rather quickly as they are usually self-assign. In my area (Western US) there are always Cafe Zupas shops, and they pay generously + reimbursement. The reports are a bit involved, but nothing too crazy. I've done enough of them that my modus operandi is well-established and I can crank out the narratives rather quickly.
I've done a few of the high-end places (Fleming's, Monaco Hotel) and, while the reports are more involved, getting a top-notch fine dining experience in exchange for a couple of hours' work if acceptable.