The #1 on M Countdown on 2012-09-20 was “I Wish” by F.T. Island — win No. 1 for the song and career win No. 3 for F.T. Island (No. 3 on this show).
“I Wish” has 1 music show wins in total: 2012-09-20 M Countdown. F.T. Island had 3 wins in 2012 up to this date, and this was win No. 3 on M Countdown. The previous week's M Countdown #1 was “Pandora” by Kara. Episode data comes from broadcasters' published results (Wikipedia lists, CC BY-SA) and is recounted daily.