Nobody voted for Intro which is amazing to listen to and wonderfully composed 
But it's understandable, all tracks are great...
So as a song I would pick Justice because it's unique, with the guitar riffs effects. One of my fav part is Dami's "Guiding me up, guiding me up", her low tone cuts so well into the prechorus and adds a heavy ambience to the song. Also, I haven't noticed before watching a reaction video and the person notified it, when Siyeon sings the first chorus "Justice makes me..", the way she sings "me" is so deep and husky, which makes me loving it even more now!
This said, the other songs are great too... Fireflies surprised me the most, I thought it would be very repetitive but it's not and it's a melodic song. 2 rings also surprised me because I thought it would be too pop for me but it was not, adding Handong's lines where she sings differently it's refreshing and catchy !
And Stomp, I didn't know what to expect and then I was again, surprised by the heavy track I even though it could be a TT (my dream is to have Stomp's bridge in Justice dsjkdjs). I especially like Sua's low voice there.
Conclusion : To not listen to the highlight medley, we never know what they are giving us!