I think you can get up into the top of the CLARO building on Av Libertador. There may be a club up there or a building that straddles Libertador & Av. 9 de Julio (the highway that feeds into it).
I have friends who have shown me pictures of Av. 9 of Julio and the Obelisco taken from from high up and almost straight on along the avenue.
Other than that, I'd try to get to the top of one of the new tall buildings in Puerto Madero maybe... but then again, maybe not.
If they ever finish those twin towers in Palermo Hollywood on Juan B. Justo, that will be a primo spot. You might have to sleep your way to the top, but it might be worth it for the photos. Then again, the view might not be the only thing that's breathtaking.