Ironically the Hollies two biggest hits in North America were He ain't Heavy and The Air that I Breathe. the reason I say this is because most of their songs were written within the group, but not these two. He ain't Heavy was originally done by Kelly Gordon and the Air that I Breathe by Albert Hammond then Phil Everly of the Everly Bros. It was after Phil Everly's effort that the Hollies tackled the song and it became a huge hit for them. Whether it became their signature song is up to debate. and by that I don't mean what we like as our fave but the one song that defines or is easlly identifiable with that group. the choices i have to chose from are Stop, Stop, Stop/ Carrie Ann/ He ain't Heavy/ The Air that I Breathe.