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.
Don't know of any top floor restaurants. There's always been plans for one in Puerto Madero (a great location for one), but nothing has been built yet. The proposed Alvear Hotel in Puerto Madero will supposedly have one on its top floor.
I believe the German Club on Corrientes has good views. The Sheraton used to have a lounge on the top floor with good views facing Retiro Station. Not sure if the lounge is still there. I believe there is a public restaurant on Corrientes with excellent views but forget its name. Maybe someone can help.
The German Club may be open to the public now. Clubs here are generally fading out. Most have opened their restaurants to the public (Spanish Club , Circulo Italiano, Basque Club for example are all open to public)
I wouldn't have described it as a lounge, as such, when I was taken up there a little while ago IIRC it was just a connecting area between different parts of that level BICBW. The view is wonderful on a clear day: if I can dig out the photos I took at the time, I'll have a go at posting them here.
My personal favourite view is when the red roofs are catching the first rays of the morning sun and my plane is descending into Ezeiza. Nothing beats that for me. It's called coming home.
Measured in years, not months or days, I'm afraid.
Much kudos to any non-Aussie who recognised that line; shame on any Aussie who didn't.