I love all their songs, but Irresistible is definitely the best. The way all their voices combine and harmonize during the chorus gives me goose bumps every single time. I almost want to cry. My favorite lyrics are '
It's in your lips and in your kiss
It's in your touch and your fingertips
And it's in all the things and other things
That make you who you are and your eyes irresistible.' At the end when Zayn comes in and does his "ah" part during the last chorus it sounds absolutely incredible. And yeah, that's by far the best song they've recorded so far, and it captures their talent perfectly.