Hi Morganmike, this is my absolute favorite restaurant in Cologne. For 35 years.
https://www.lemoissonnier.de/It had two Michelin stars. They gave them up because they wanted to lead a quieter life without the star stress they had successfully endured for 25 years. It has reopened with the old crew. Now as a “normal bistro” haha. The cuisine is just as good as it was when it was starred. The chef is Belgian, some people say that the best cuisine is French, but the best chefs come from Belgium.
If you prefer traditional Kölsch food, then I would recommend the Päffgen.
http://www.paeffgen-koelsch.de/A brewery with its own Kölsch beer and very tasty Kölsch cuisine. It's more for a boisterous and merry evening, but unforgettable.
If you are interested in music, the Päffgen is not only the most famous brewery in Cologne. There was a daughter who became a singer. Nico.
https://en.wikipedia.org/wiki/NicoAmong others “The Velvet Underground” singer.
Whatever you prefer, it is highly recommended to book well in advance, especially for the Moissonnier.
I would like to add. 35 years ago, I was a student at Cologne University with little money. But at the Moissonnier, you could get the dish of the day with a glass of very good French wine for 18 German marks, today it's € 9. I used to go there once a month with friends. It had just opened. That's how times and prices change.