Wanted to share a few recommendations from a recent trip to Buenos Aires. Overall, I think you're hard-pressed to go wrong. If you want to see some pics of the places/food written about below, visit: http://erinskitchen.blogspot.com/
Both at Cabana Las Lilas in Puerto Madero and La Cabrera in Palermo. Would recommend reservations at both places, but easy to get the day of. The best filet mignon of my life at Cabana Las Lilas--bigger than my fist, the definition of tender. La Cabrera is cheaper but also good, and you get a dazzling array of tiny dishes with your steak--roasted egggplant, marinated pearl onions, applesauce, marinated garlic cloves--it's truly a wonder.
Stop in any one of the Havanna cafes, a local chain, for great alfajores--sandwich cookies filled with dulche de leche and covered in chocolate. They also make a great lemon cream sandwich cookie--tastes like meyer lemon to me. You can buy boxes to bring home!
I didn't have a bad empanada anywhere, though some crusts were better than others. Definitely pop into neighborhood joints, but also try the famed La Cupertina in Palermo on Cabrera, especially the sweet corn empanada. Best crusts--from a woodfired oven. Just under 60 cents for one. Closed on Mondays.
Homemade hard cider at many a cafe/bar. Some of the best was at Bar Federal in San Telmo (the food there wasn't anything special, however). Light and refreshing, a great way to cool off after a day of pounding the pavement.