I don't normally eat at theme parks when I'm on a park trip because it's pricey and I don't like feeling bloated and uncomfortable which tends to happen after I've eaten. Usually I just try to eat before and after a park, but the odd time I'll get a snack or a meal if I'm super hungry.
Six Flags is by far the worst. The only good thing is that they have Panda Express. Way overpriced, and it's worse than McDonalds food. Cedar Fair is really no better but I do enjoy the fabulous cream cheese stuffed pretzels they have at Cedar Point <3 Usually when I go there I try to have Famous Daves which is right outside the park. I have to say, the buffets at Cedar Fair parks are usually quite good. Unfortunately Disney is really not a whole lot above Six Flags and Cedar Fair, with their cold and subpar buffets and their gross cheeseburger macaroni and cheese.
I think a lot of the Southern parks and the one offs/small chains have the best food. Dollywood had a great buffet, Silver Dollar City has those skillet things that look, smell, and taste amazing, and Holiday World has some awesome choices in Thanksgivingland. Busch tends to have good food as well, but it can get really expensive.
