The best ones I've been to in the past six months or so are
Crizia -- Gorriti at about Thames
Fantastic menu, slow roasted cordero, oysters, fish, but the wine list is expensive. We went for our anniversary, be prepared to drop some cash but have a truly wonderful meal.
Chila - Puerto Madero
Haven't been since last sumer but everything was very fresh, presentation very nice, i seem to remember a good drinks list.
These are both expensive options (for those on pesos), as far as best restaurants for a lighter wallet I can give other recommends if you like.