I wish this was still the case, but in my experience the differences between individual fast food joints is enormous. We've got one Taco Bell in town for example which is known to take 10+ minutes per car and they constantly screw up orders. This usually shows up as a 1 star fast food restaurant rather than the more common 2-3 stars on review sites.

For me the tell is their soda. My guilty McDonald's pleasure is their Coke mix. Something about it tastes much better than other restaurants or bottles. Except at some McDonald's where they seem to have the CO2 cranked up too much. I can't even get a consistent soft drink from McDonald's anymore. When I get unsalted fries though... those are basically inedible.