For nice looking and always reasonably tasting food, the easiest choice is a restaurant in the Palermo area. Palermo is huge, start with Palermo Viejo. The four-five blocks-square area around Plaza Serrano (Cortázar) is a culinary hot-spot, many good places to eat, along with lot of local designer-fashion clothing stores.
The food is all designer food, all too similar in tastes, just like any fancy (or pretentious) restaurants in the US. But similar dinners in the US at fancy restaurants will be far more than the US$15-25 per person that they cost here. This is still expensive, but worth it to try something more than just grilled meat day-in and day-out when in Buenos Aires. Most places are good, most food is the same, so drop into a place that you like the look of, plenty of information with pictures on the web.
Palermo has numerous restaurants, there is one good one you could try every day for a month. In fact the Restaurantes en Palermo at Guía Oleo guide had information and customer comments on 731 restuarants in March 2008. Some may have closed and information not updated, but still a very large list to work with. Or, just walk around the Plaza Serrano (Cortázar) area, or one of the restaurants listed abthough that too is a large area to work with, it is a good place to start.
To detail my experiences in two - major thumbs-up for Cluny, and a somewhat-minor thumbs-down for Social Paraiso.
Cluny at El Salvador 4618 (corner Malabia) is a huge place with many non-beef dishes, and excellent beef (lomo) too. And somewhat of a shocker, incredibly good service with great attention when I ate at the bar, and at the end, there was an incident that the manager overheard and on his own immediately resolved it in my favor. Such level of service is a rarity in Buenos Aires, so this was a very good overall experience in a very nicely designed restaurant. Afternoon meal packages went for AR$40 to AR$48 (US$15), add around AR$20 for evening dining.
Social Paraiso at Honduras 5182 (corner Uriarte) is a less expensive place, but they have good food. Service on the other hand is somewhat high-and-mighty. Since I can rarely eat both a main dish and dessert in one sitting, I returned another day to just eat dessert since the descriptions looked interesting. But the person (who may have been the owner or manager?) who opened their always-locked restaurant door told me that they do not serve desserts only, it is only available if I order food! What a strange concept, it is a very small restaurant, but was half-empty, yet she still finds it perfectly alright to turn away someone interested in eating there! The cost of a dessert plus water would have been nearly the same as their afternoon-meal packages, so the cost should not have been an issue. All in all, a pity, but then such are the customary whims of those who purvey "designer" food.
So thumbs-up to Cluny on multiple counts, for food and service and decor, and a thumbs-down to Social Paraiso for their whimsy service.