I'd go for the Brown Derby. We've eaten there, Sci-Fi and 50's prine time and HBD had the best food, by far. You can't miss with the Cobb salad and the grapefruit cake!
Sci-Fi and 50's Prime Time are two of my favorite places to eat and I've yet to have a bad meal at either. we've tried all the TS places at DHS and those are the two we always go back too.
We really like the Hollywood Brown Derby. The steak and salmon were very good and the Cobb Salad was large enough for 3 of us to share. I didn't care for the grapefruit cake, but it's worth trying at least once since it's iconic.
We've not eaten at 50's Prime Time or the other TS restaurants in HS though....so we can't really give a good overall of the 'best' place to eat at that park.