Just came back from Argentina and wanted to share some feedback with the rest of the foodies.
First of all, thank you to everyone for the recommendations, as most of them were very good!
La Cabrera - touristy, but excellent meat and friendly services. nice location in Palermo Soho. very reasonable - around $70US for two with wine.
Don Julio - similar to La Cabrera, but a bit more rustic and slightly cheaper. Very comfortable recommending this place for a casual dinner.
Cabana Las Lilos - a bit fancier than the other two. located on the water in Puerto Madero. Also excellent meat, complimentary appetizers and dessert. Prices are 20-30% higher than La Cabrera/Don Julio. Monster wine list!
Osaka - one of the best non-meat dinners! Japanese with Peruvian twists. serious sushi buffs might be disappointed with limited selection of fish (tuna was not available), but we didn't mind. the place was happening and not touristy at all. this dinner was a bit pricey at $130US$ for two, including 1 beer and 1 martini (no wine).
Club 647 - having read rave reviews, our expectations were quite high... the major issue with this place was the fact that it was EMPTY (1 more couple in addition to us in a place with 100+ guest capacity) on a Saturday night around 10-11pm. this was very odd, considering that this is supposed to be one of the best restos in town. food was OK, but very forgettable.
Piegari - excellent Italian place in Recoleta (near the Four Seasons). portions are meant to be shared. highly recommended.
Park Hyatt - we went for lunch and thought it was a good spot: both food (tuna tataki, sushi rolls) and atmosphere (we ate outside, in a very nice garden) were top notch. As one would expect, prices are quite high.
1884 - The best food overall (try mustard-topped steak) and very impressive setting, but we expected better service from a place of this caliber. Quite expensive at $130US for two.
Ruca Malen winery - highly recommended. we stopped by for a 7-course lunch with wine pairing as part of a wine tour and were thoroughly impressed by food and wine selections.
Cavas Wine Lodge - stayed there for 2 nights. perfect honeymoon spot (private house set right in the middle of a vineyard, with your own plunge pool and a terrace with a fireplace). fantastic sunsets. Very expensive, but definitely worth the money!