Thinking Out Loud probably, but I could see it being blocked by UF. If so, then probably Style, but I see some random song snatching a few weeks in between. Lips Are Movin perhaps, but I feel it's a bit too late.
Thinking Out Loud during Grammys week if he performs it, Style maybe during Grammys week if she releases the video at midnight after the Grammys on Monday, or Katy if she releases a song after the Super Bowl at midnight in Monday along with its video.
Edit: Super Bowl is before The Grammys. So I feel Katy can debut at #1 with a killer performance on Sunday, then start a Monday radio deal, and the release it to iTunes and drop the Music Video at midnight after the Super Bowl for a full tracking week of sales and streaming.