From a service stand-point we found MM's to be the best of the 3, on a trip last summer. Our filter is that we have a DS who has anaphylactic allergies to even traces of milk products or to any nut exposure. As you mgiht guess, this requires advance prep, then extensive conversation with the server and chef on site. Brown Derby was very competent, but the server was kind of snooty about it, and trying to get a dessert he could eat, even with advance prep, was a pain.
At H & V they acted like the special care we have to take with the buffet was a royal pain the rear. The only special thing we'd asked for was either a plain baked potato or rice to available and ready when we got there and it felt like we had asked for the moon. For dessert they came up with some rice dream, which my DS absolutely hates, and made him feel (on the floor, with people watching) that he was being ungrateful, even though people who can eat normal foods often have some dessert items they like and others they don't.
But at MM's they jimmied the package to make him a special flatbread without cheese, they checked with us 3 times on ingredients; they did a special salad for him so the rest of us could eat the normal one, and they came up with a dessert which was tasty to him. It was very attentive, did not call attention to him, and he came away feeling "normal" and nurtured.
The food at BD was probably better, but MM won in service, hands-down.