We've always enjoyed our meals at Mama Melrose's *yes, it is sit-down* and SciFi. Although SciFi's cars are a little difficult for a large group *you would all face forward, like in a real car, so that makes it hard to converse* I think they still have tables, with the front of the car at the head of the table, and the back of the car at the end. Not sure how many can be seated at these tables.
Hollywood Brown Derby is our very favorite restaurant in MGM, and one of our top 5 in all of WDW. If you want a little more upscale, this is the place. Not that it is swanky, park attire is still acceptable here, but the food is a cut above *or two, or three!* the other sit-downs in MGM.
If there is a mix of adults and children in your group, I would go with '50's PrimeTime. Good choices, comfort food, great shakes and a fun atmosphere. The kids especially will get a kick out of the waitstaff's antics, as it usually comes at the expense of the adults!

My kids are teenagers now, and they still talk about when the waiter made Dad eat his green beans!