We just returned from BA and had terrific steaks, everywhere. Not a bad one in the bunch and all were cooked to order; "jugoso" as medium rare. Our "favorite" spots were:

La Cabrera - everything was great, including a half order of sausage to start. However, they've changed the way they cook their provelta, I think I preferred the former method. Overall the spot is superb, and very crowded. We arrived at midnight and the restaurant was packed.

La Brigada, on Pena. Excellent also. Yet, some seemed to like the entrana better than the bife de chorizo. Good service, enjoyable atmosphere, great salads too.

El Establo. Some thought the steak here had the most flavor. A bit less expensive than some of the other spots. Good atmosphere, nice mixed salad and very good service.

Also, don't miss El Sanjuanino, on Posadas, for absolutely the best empanadas we've had anywhere, though there's a debate, as always, as to where the absolute "best" empanadas can be found in BA. About $1 an empanada and we ordered more than a dozen for four people. A great spot.

Also, the wines lists in BA obviously have more Argentine selections then you will ever find elsewhere. Many from vineyards I had never heard of. If there's a sommelier at the restaurant, or someone who knows more than the waiter, he is worth searching out.
Have fun. BA is a terrific city.