Nothing matches the meat from the Pampas grass-fed Argentine cows, and that meat is the focus of the dining experience throughout the city, from the humblest parrilla (grill) to the finest business-class restaurant.
As there are almost as many Italian last names as Spanish ones, it's hard to call those of Italian descent a specific ethnic group with Argentina as you would in North America. As such, Buenos Aires's Italian food is Argentine food in essence, and pastas and other Italian dishes are usually folded in with traditional Argentine offerings such as grilled beef.
La Boca is Buenos Aires's historic Little Italy, the place where Italian immigrants first settled at the end of the 19th and 20th centuries. The atmosphere in these restaurants plays on this past but it's not where the city's best Italian food is served as it caters to the tourists mostly. Instead, it is usually found in old, simple parillas that have operated for decades and include pastas on their menus. Empanadas, dough pockets filled with minced meat and other ingredients, are also an Argentine staple offered almost everywhere.
Portenos eat breakfast until 10am (my kind of people!), lunch between noon and 4pm, and dinner late- usually after 9pm, though some restaurants open as early as 7pm. Executive much menus (usually fixed-price three-course meals) are offered at many restaurants beginning at noon, but most dinner menus are a la carte.
Though Buenos Aires is a very cosmopolitan city, it is surprisingly not a very ethnically diverse place, at least on the surface. However, the influences of Middle Eastern immigrants who came to the city in the wake of World War I are reflected in a few areas. Middle Eastern restaurants are clustered in Palermo Viejo and you'll find many kosher restaurants (some traditional, others recently opened by young people trying to bring back the cuisine they remember their grandparents cooking) along Calle Tucuman in particular. Because many Buenos Aires Jews are Sephardic or of Middle Eastern descent, you'll also find Arabic influences here.
With a renewed definition of what it means to be Argentine, native Indian and Incan influences are also finding their way into some Argentine restaurants, as well.
Lastly, but certainly not least(ly?), dessert is ubiquitous. This is the one of the city's strong imports of the French immigrants. It's easy to be quickly bowled over by the pies, pastries and fancy sweetbreads found on the other side of windows in many of the main street's cafes, restaurants and bakeries. It's not uncommon to find sugar-filled delicacies in Latin America writ large and Buenos Aires is no exception. It seems to be fair game to throw down on one of these treats at any point at the day or night and...who am I to argue? When in Rome...