For the money (when reasonably bonused, obviously), quantity of shops available and relative ease of reporting, I'd say McD's.
Strictly talking about food, my first choice would be Pita Pit, although the narrative requirements and relatively low fee make those somewhat less exciting. I also really enjoy Cinco Chicos' veggie sandwiches, and those shop reports are reasonably fast and easy so there's a good balance there.
I guess if I'm picking just one, I'd have to go with McD's.