Probably some of the classics from Mozart or Beethoven, but chances are aliens will have picked up all our crappy radio signals decades ago and that means we're fucked! Think of all the crap that's played on the radio compared to the actual good stuff.
"shine on you crazy diamonds" from Pink Floyd!
or something completely different like jamiroquai.
AT THE LEFT HAND OV GOD!
This video is monolithic!something from Jason Becker or Devin Townsend ... the aliens will relate quicker to one of their own
Morbid Angel - Where The Slime Live
A classic!