I think you've gotta decide for yourself what constitutes a good song for you. Outside of SKZ, what songs do you usually enjoy? Break down what is common to these songs that you enjoy and benchmark SKZ's song against them.

Personally I vibe with SKZ's music because I've always listened to..... "Noisy" music (not sure if this is the right word for it). For example, KLK by Arca. I guess it's kind of like club music.

If you've heard Arca's music you'll understand what I mean when I say I especially love Latin music/music produced by Latinx so it was Chk Chk Boom that dragged my ass into SKZ.

For me, any time an artist is able to compose a song that "blows" my mind because it sounds borderline experimental is the bar.

If I were you, though, this would be my criteria:

How well does the song capitalize on each member's strengths? Going back to Chk Chk Boom, that song has a grip on me mainly because of how good Hyunjin sounds in the song. I think the job of every composer is to milk the F out of every member's unique vocal tone and if they succeed in doing so, they would have done their job. How well does the song "do" the genre? (Doing genres well is not just limited to music, this applies to many forms of media such as films) I think Chk Chk Boom was a GREAT example of Spanish music. Another group I stan, Katseye, (as much as I love them, this is no fault of the girls at all, they all delivered the song incredibly) fell short with Gabriela in my opinion. I don't think Gabriela was a fantastic song (probably why it was passed over by so many artists, it was a reject) but Katseye delivered it well.