Adele or Gaga if their lead singles are huge (Gaga would need a bigger lead though), Taylor almost certainly, all depending on who releases first of course. ARTPOP should be out first but... you can't be too sure at this point
I wouldn't put it past someone unexpected, though. I remember a few months ago there was a similar thread and of course at the time we didn't know JT was releasing a new album, let alone one that would sell so damn close to a million.