I had to really think about this one. We have a little (drive through only IIRC) dive in a REALLY bad part of town with killer burgers. IIRC its next door to the cop shop so if you arrive safe you are fine.
The White Barn. Burgers are pretty big
https://www.yelp.com/biz/the-white-barn-saint-louis
My fave though was in my home town at a little pool hall called the Blue Ribbon. Nothing really fancy about them other than the super cool old bar i could sit at even at 7yrs old. Just grilled onions on a cheese burger with a hand dipped MALT...not a damn milk shake...and they put the steel mixer on the table with your malt!!!
Now its called the New York bar and grill. It has been for probably 20 years or more but ive never even went in the place.
https://www.facebook.com/New-York-Bar-And-Grill-292860747112/