Moosburger Lodge is currently the best restaurant available at Six Flags St. Louis. The building it calls home once housed a McDonalds, but now the only McDonalds can be found right outside the park next to a gas station across the street for Six Flags' parking lots.
The food is perfect at Moosebuger Lodge, especially their fries! Making the experience even better, the themeing inside the restaurant beats the themeing that is set up at many of the rides throughout the park.
This is definitely the best option to eat at if you plan to eat inside the park.