Infinite
Thanks to friends, my only impression of K-pop was Exo and BTS for a few months, which left me with a very limited impression of what it was and therefore disinterest in it. After watching High School Love On, I curiously looked into the one group both boys members of, and THIS song presented a completely different vibe from what I expected of K-pop. Listened to it from then on, starting with them. I guess that was somewhere in 2014 or 2015.
Mind you, I don't even listen to Infinite anymore ^^' Still love their songs though.