While not their most iconic song, I think f(x)'s Airplane is the song that I've seen cited a lot in terms of the group's best song.

Namely, not just because of the cool "reverse house" backing track, but every member gets to shine vocally and gets ample time to showcase why each member is important to the overall f(x)'s "sound".

Sulli and Victoria get time to shine on choruses (Sulli even leads on the second half of the first chorus! and Victoria!! leads the 2nd half of the second chorus!), Amber sings in her lovely alto range and gets more than eight bars to do so, Sulli's high harmonies, Victoria's low harmonies, and Amber's alto harmiones blend so well in the chorus when Luna/Krystal lead, and both Sulli and Victoria actually flex their vocals instead of doing the "cheerleader chant" style bits they do in most of f(x)'s tracks.

I always cite this song as their magnum opus. They've never been able to top it in my eyes because the song is pure perfection.

Also, still so bummed we never got the only f(5) concert on DVD. This fancam is legit though.