I've dined 8 time at DHS: 3 times at 50's Prime Time, 2 at Mama Melrose, 2 at Sci Fi Diner, 1 at Brown Derby.
Brown Derby was really overpriced and overrated, IMHO. Sci Fi Diner had a cool theme, but it wasn't fully realized and I found the menu selection lacking and the food quality poor on both visits.
Mama Melrose provided us our absolute best meal at DHS ever on one visit, and our worst on another. Food was acceptable both times, but one time our server was fantastic, and the other, he was miserable.
50's, IMHO, strikes the perfect balance. The food can be very good, the theming is dead on, and I personally find it one of the best dining values in all of WDW. The sampler plate (meatloaf, pot roast and fried chicken) is really, really good. And I love pre-dinner cocktails in the lounge.
Personally, if I was only having one meal at DHS (and I am on this upcoming trip), it would be 50's Prime Time Diner.