in general, probably orange caramel's catallena. it always leaves a lasting impression!
but usually i try to pick something i think they might like/related to their tastes or interests (ex. a friend of mine likes detective stories/sherlock holmes so i showed him shinee's sherlock since i thought he'd get a kick out of it).
I would show them "Killing Me" by iKON. It has a good combo of vocals and rap and it doesn't really lean heavily to a softer or darker concept. iKON also has very different vocal tones than most kpop groups so I think that would be inmportant to show. Also the mv is amazing and really well done.
Starry Night by Mamamoo, I personally really like it and I feel like the people I know would like that type of song too. Also, the mv is so aesthetically pleasing, it might help them like it even more.