ianBK4, you have to keep in mind that Bey is in the spotlight right now (H&M, tour etc.) so ofcourse Bey's songs will chart in more countries than Gaga's songs since Gaga is pretty much hiding. But yes, Halo has generally had more longevity on Itunes than Telephone. And no, SL did not sell far more than Telephone. It probably hasn't even outsold it yet if we go by IFPI's numbers.
I think the answer is either Single Ladies or Halo. Voted other though