You guys are naming some restaurants that are chains you can find pretty much anywhere.....
But for some one-off Cincy only choices -
Izzy's is pretty much a good choice, make sure you get a potato pancake with your Reuben
On the west side, Price Hill Chili would be a good choice - get one of their breakfasts with goetta, big flat slabs, any time of day....or one of their sandwiches. Very interesting place.
Also on the west side, Maury's Tiny Cove is a restaurant where you can get a great steak in an atmosphere that'll make you feel like Tony Soprano might walk in at any moment.
Also, look across the river in Covington - some good little restaurants over there.
Already mentioned, Montgomery Inn is a no-brainer, along with the Precinct.
If you're looking for a fast lunch, any of the Penn Stations are good (although you can find that chain elsewhere too, so it isn't as special).