Btw, this must sound ridiculous to most of you, but I used to live in Frankfurt as a kid for some years due to my dad being posted in Germany as an export manager. This was in the late 1970's/early 80's.
So I still love this undervalued city, actually everything about it. Only Cologne rates higher in my books, due to studying management there later.
There was a lovely mom'n'pop style Delicatessen close to Frankfurt Railway Station earlier. Unfortunately the couple that ran it have retired and their kids only do online wine retail now. Before, it was par with anything New York City or Paris could offer.
These days, I like to stay overnight in Frankfurt-Niederrad. And one of my Frankfurt layover highlights there is an "Imbiss" owned by a very sympathetic local couple who, if they were Italian, would surely operate a perfect Trattoria.
Now they are doing a sublime Rindswurst, pommes frites, salad and a soft drink well under 10 euros. Love this place located in front of the local hypermarket a few minutes from Niederrad S-Bahn station and regularly eat there instead of the city or overpriced FRA food outlets.