It was Wolf by EXO :)
Super Junior - Happiness
SS501.....love ya 
that was first korean song that I listened , I really love this song
I heard a few songs before I got obsessed, but what dragged me into the K-Pop world was Big Bang's Monster. <3
My first kpop song was SJ-Bonamana
My first K Pop song was from DBSK Mirotic. And it was a new experiences.
So I searched more of them, and I found it really catchy.
And later I started listening to other K Pop groups. :D
My first Kpop group was EXO with the song Growl... I really loved this song. :3
For me, it wasn't a kpop group, it was a kpop artist... and it was Taeyang with his song Wedding Dress. I found it on YouTube when I searched on "wedding dress" because I just wanted to se if I spelled it right, I thought his video pic looked interesting so I just clicked on it... I didn't know it was a MV and I didn't know about kpop before I watched it. I'm now very thankful for that and I've listen to kpop for over 5 years! ^^<3
Actually it was a Japanese song from DBSK, Love in the Ice. I saw a few live videos on youtube and then I started to follow the group.
f(x) - hot summer I guess 
For me it was Super Junior , it was 5 years ago since i've heard their "Sorry,Sorry" in "You'r beautiful" and they led me to the K-pop world :D

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DBSK's Rising Sun (and the rest of that album <3)