Shinee "Replay" is the best KPOP debut song

I would say its unpopular because a lot of the newer Kpop fans are not too aware of the impact that this song has made on the industry.

It might also be due to personal preference because Shinee got me into kpop in 2008. To me, this song is almost flawless and is amazing in the sense that it was one of the first major Kpop songs that really gives off a western vibe while still retaining that kpop feel.

Other debut songs that I think come close are SNSD's Into the new world, Miss A's Bad girl Good girl, Exo's Mama, and Blackpink's Boombayah.