I came into this thread ready to say if it's not Replay you're wrong. Someone beat me to it though. 7th Sense is also a bop though.
All debuts mentioned above are amazing. So I can't choose. But WINNER's EMPTY is such a mood. I have been obsessed with that song for a while. That song just never gets old.
the 7th sense is the only kpop song that doesnt sound kpop, the song felt so un manufactured, it's a god damn masterpiece, from the arrangement, music, lyrics and mood. that goddamn bass at the end. ugh, pure eargasm. i was so shocked this came from a kpop group when i first heard it. it's definitely not kpop fandom/public friendly but man it is actual art and it still hasnt been topped and i dont think will ever be. it was an anomaly, but the best happy accident.
MCND is close for me also ofc Pirate king, Replay, Empty, even Rhythm Ta and My type... (also Exo mama is a bop but everyone too tasteless here to talk about that)