I agree with Cabana las lilas for steak. Start with a "provoleta" to be shared at the table (they can split it 2/3/4 ways) it's a chunk of grilled (yes grilled) provolone.

Besides steak, Argentines excel at Italian food -- but you won't find red sauce or meatballs very much. I like Piegari, which is in the Posadas underpass under Av. 9 de Julio, next to the Four Seasons Hotel. Specialties include: Sorrentinos alla Francesa (big raviolis stuffed with Italian ham and cheese); Raviolis alla Scroffa (little spinach raviolis with a pesto sauce); Fideos de Morrones (Red-Pepper fettucine with a fresh sweet pepper-tomato sauce); they also make a good pizza. If you have to wait for your table, walk straight back to the stand-up bar. The girl there will pour you some champagne.