It would have to be something REALLY obscure ---- because between 6Music and the dark regions of Radio 3 pretty much everything I have has been played on the radio in some form. I could choose something by Einsturzende Neubauten as they don't get a HUGE amount of airplay, or I could choose something heard at a gig (such as Jeff White or Felix) or something produced by friends (Rob McMinn or Spunk).
In the end I'm selecting something from the majesterial Godspeed You! Black Emperor. Because they don't get played on radio enough.