For a traditional experience, I'd suggest "La Chacra" on Cordoba just off the 9 de Julio. Terrific, authentic food and service.

You might also try "Aquellos Anos" which is out on the Costanera highway near the downtown airport.

Either way, in addition to the beef, there are also a lot of other equally-excellent things to choose from. My personal favorite is the matambrito which is a pork tenderloin, flattened out and grilled and served with lemon juice. In addition, the salads are always excellent in these restaurants and very nicely complement the meat.

Red wine is always appropriate, as well.