Over the past few years, redeveloped Puerto Madero [1] has had an impact on the restaurant scene, but more because of its overall tourist appeal than its quality—with a couple of exceptions, the food is unimpressive.
Most restaurants, though, have outdoor seating along the yacht harbor, and when the weather’s fine it’s ideal for people-watching. Because the area draws foreigners from nearby luxury hotels, restaurants here are accustomed to dealing with dinnertimes as early as 7 p.m.
Opposite Dique No. 4, the best parrilla is 
Cabaña Las Lilas (Alicia Moreau de Justo 516, tel. 011/4313-1336, www.laslilas.com [2]). Often packed for lunch despite soaring prices, it offers complimentary champagne and snacks while you wait; its bife de chorizo (which has skyrocketed to US$30) may be the finest in town.
Puerto Sorrento (Alicia Moreau de Justo 410, tel. 011/4319-8731, www.sorrentorestaurant.com.ar [3]) is the best seafood choice, and at midday it has an excellent three-course almuerzo ejecutivo (US$18 including soft drink, beer, or wine). The service is exemplary.
Links:
[1] http://www.moon.com/destinations/argentina/buenos-aires/sights/puerto-madero
[2] http://www.laslilas.com
[3] http://www.sorrentorestaurant.com.ar