I had dinner at the Piano Nobile in 2010 and it was crap. The worst sushi I have had in my life. My experience at the Gioia was a bit better but nothing memorable. The Duhau Palace is not the kind of place you go to get the best food in town. It's the sort of place you visit because you are looking for refinement and good taste.
Good food? I had my best dinner in a restaurant called Contraluz this year. It's inside the Alvear Art hotel. The restaurant is always empty in the evening, boring boring boring, the decor is even worse but the food was excellent. That was a few months ago, you know what these things are like... they change very very quickly.