Isn't it, just?
I think Piazzolla's hugely underappreciated; though perhaps not underrated, since he's highly appreciated critically, just not broadly known by the general public (except in Argentina, where he's understandably a national icon). While it's far too soon to judge the long-term legacy of a composer who died in 1992, if there's any justice he'll be remembered as one of the giants of the second half of the 20th century. He singlehandedly transformed tango from a largely geographically specific popular music into a classical musical form of global relevance, fusing tango, jazz, and the classical training he received in Paris under Nadia Boulanger.
His two most famous pieces are probably Libertango and Adios Nonino (latter link with Piazzolla on bandoneon), but Maria de Buenos Aires, his 'tango operita', is likely his masterpiece.
If anyone's more curious:
http://en.wikipedia.org/wiki/%C3%81stor_Piazzolla
