Hi!
in did, doesn't exist a "skyscraper" with a public platform. But some high-rises has a café or restorant in the top floor.
1. 23th floor of the Panamericano Hotel, 551 Carlos Pellegrini street. One and half block from the Obelisk. Here you have to order at least a cofee to stay.
From there you can almost touch the obelisk, and it gives a 270 degrees view.
2. The Comega building, 222 Corrientes avenue 19th floor, A222 Restorant. It's a twenties racionalist building. There is a cafe and in the night a restorant. From there you can see part of the finnancial district and Puerto Madero, the newer and exclusive neigborhood.
3. The "Palacio Barolo" maybe the most fascinant building to visit. There are guided tours through the building and finaly you can see the city from its beacon tower at 100 meters. I let you the website:
http://www.pbarolo.com.ar/homein.htm
If you like to view the city from the highs I recomend you to do a Helicopter trip over the city. The cost is from 150 to 200 USD each. But for that, I recomend you to go to a Turist Information Point in the city. I don´t know too much.
Also Buenos Aires is a city to walk, the most fascinating and richest architecture is low-rise.
Well, if I can help you in something more just post.