I don't think Shoutout To My Ex is their best track, however it is interesting as it allegedly concerns the break up between Perrie Edwards (of Little Mix) and Zayn Malik (ex One Direction).
In my opinion their finest moment to date was a mash-up of their two best hits Love Me Like You and Black Magic performed on The X-Factor last year. Both great pop songs in my opinion.
www.dailymotion.com/video/x3epg2e