Do you really consider that a beautiful building? I believe they demolished an old mansion or some kind of gov palace to build that weird structure (perhaps it collapsed unintentionally, or a fire or something, earlier on, or something like that, my memory has seen better days). I still like it somehow, but I do not consider it beautiful like most of the Paris/Madrid style buildings people are describing in this thread.
Anyway, I can add to what's been said already that I am quite fond of the Puerto Madero skyline, and I usually don't really enjoy modern architecture. Especially the Parisian style 'skyscraper' looks pretty original to me.