Excellent beef... Isn't that why you're visiting Argentina, after all? OK, OK the Tango is actually why I visited, but few will deny that the meat from the pampas grass-fed cattle is just delicious there!

I liked Cabaña las Lilas. A lot. It is a carnivore's paradise.
Address: Alicia Moreau de Justo 516
Phone: 4313-1336

Oh yes, while in BA, I'd also recommend The Faena Hotel and Universe - even if you're not staying there, go have a drink by the pool there and check out the Starck-designed property.
website: www.faenahotelanduniverse.com

Chloe
Before giving an answer, I notice that you asked about "interesting" restaurants, not good or excellent or fancy restaurants. What's exactly what you are looking for?

Cabaña Las Lilas is very popular with tourists (which is why it is always so crowded), but locals who know their steaks avoid it. The atmosphere is a bit wild and the food and the service is spotty. But tourists just seem to love it and it's in all of the guide books. Also, the restaurant was also sold a few months ago, so it's actually gotten worse from what I've heard.

For steak, Patagonia Sur located at Rocha 801 (tel. 4303-5917) is a terrific choice. It's pricey for Argentina, but definitely worth it.

Sucre located at Mariscal Jose de Sucre 676 is a stylish modern bar and restaurant that serves excellent steaks and tapas. Terrific wines and very cool atmosphere. Don't miss the suspended catwalk that takes you above the action on your way to the bathrooms. Their telephone number is 4782 9082. Reservations are a good idea.

Yeah, it's touristy... We knew that going into Cabana Las Lilas. Yet, our experience was delightful, and the beef we had there was better than my steak in Peter Luger (NYC), which is saying a lot.

Judging by the above review, I guess Cabana Las Lilas is a "hit-or-miss" type establishment.

There are two great sites to find out about restauratns in buenos aires .
One belongs to Buenos Aires Stay, a flat rental agency that happens to provide excellent finormation on all activities and neighborhoods in BA. This a link to their restaurants in Recoleta : http://www.buenosairesstay.com/recoleta-buenos-aires-restaurants-resto-bars.htm
If you want to see the other neighborhoods you can click on the on the left and then click to see restaurants. This site also has info on bars and clubs.
The other site is www.guiaoleo.com that has a good search engine for all restaurants in BA. The main reason I prefer to use the first site is because they have links to the websites of the restaurants and guiaoleo does not. Also the first site only lists the very best restaurants in the city and guiaoleo lists them all.

If you want to eat with the locals, head to Bar Restaurant El Obrero in the Boca neighborhood. Not only can you get good meat, but expect to get a real blue collar experience.

I just returned from 3 weeks in Argentina and my brother lives in Buenos Aires. I have eaten at Cabana Las Lilas - and I have to say it should be avoided as it is an overpriced tourist trap. Puerto Madero in general is, IMHO, a giant tourist trap.

For an absolutely enchanting evening, with some of the best meat and contemporary Argentinian cuisine you will ever eat, in a divine setting, you simply must try 647 Dinner Club (http://www.club647.com/en/restaurant.asp). It is upscale, sexy and on the CNT Hot List.

I also encourage you to check out the Palermo SOHO area if you are into more trendy fashion-forward neighborhoods.

Cabana Las Lilas--great steak, reasonably frenetic atmosphere, but they tried to rip us off by charging a bottle of wine we hadn't ordered, so check your bill carefully. I wouldn't avoid it because of tourist reputation. The variety of beef cuts is worth the visit.

Our best meal in BA: Oviedo, very classic, proper, lovely restaurant with the freshest seafood (a rarity in Buenos Aires). Perfect unobtrusive service.

Second best meal: a fabulous chorizo sandwich (choripan) from a truck opposite the BA airport--go for it, because it hits the spot after a long flight.

Casa Saltshaker (extremely hyped private restaurant) : food wasn't good, service was bitchy and they were stingy with the wine, but we still had a great time at a dinner party with people we had just met.

Stay at the Park Hyatt Palacio Duhau! It's a splendid, luxurious hotel with beautiful breakfasts, extremely comfortable rooms, great outdoor terrace bar. You won't want to leave.

I'll second the vote above for Sucre. We've been there a few times now, including this past weekend, and it's always been great.

I haven't tried it myself but I keep hearing that Casa Saltshaker is over-hyped and disappointing, and yet it always gets good reviews from Time Out and the foreign press. Don't you hate that?

I love Sucre too. Other favorites of mine are (and admittedly I've been travelling for the last year so hopefully they're still there!):
* Resto on Montevideo 938 - contemporary Argentine cuisine, woman chef, tiny but incredibly popular place, in the same building as the architects association or something, so naturally attracts lots of architects and interior designs - had some of our best meals here

* Brasserie Petanque on Defensa 596 - this is a wonderful buzzy place that is always full, fantastic service, owners are just lovely, great food, a combo of classic French and Argentina - lomo a la Béarnaise is amazing if you're a meat lover!

* La Brigada on Estados Unidos 465 - this is where the best meat is in BA, comes from owner Hugo's own estancia - you can cut it with a spoon! - and you have to eat all the offal, intestines, etc, too - delish!

* Gran Bar Danzon on Libertad 116 - very stylish, buzzy bar, attracts the suits, some prefer just to drink here, but we also love the food - contemporary Argentine fusion I guess.

* Brasserie Berry, on Tucumán 77 - very French-bistro in feel, Lyonnaise food

* the restaurant at Esplendour hotel on San Martín 780 - very contemporary and chic and wonderful food, was a favorite with lawyers/business people from that neighbourhood