I believe it was Haru Haru from Big Bang that really kick started my KPop obsession.
2NE1 with their debut song, "Fire"
SS5001 was the first group I stanned. I think it was Deja Vu that had grasped my attention. And then I got introduced to shinee, super junior, cnblue and ftisland. And I guess I had lost all hope around that point. 
F(x)'s "Danger," although SHINee's Lucifer is a close second. I'll mentiono f(x) because it was the first song I downloaded. 
First song was probably: H.O.T - A Song For Lady
BigBang's Haru Haru for me but then there was a phase where I stopped listening to Kpop. I became truly obsessed after listening to BigBang's Love Song :D
BTS- Blood sweat and tears
While I knew a little kpop like just over a year before then, I didn't get into KPOP until I watched Super Junior's Full House (TV Show). At this time I got really into their song U. But yeh, variety shows and shiz is whut got me into KPOP , more than the music xD
SuJu always holds a special place to me tho. 
Taeyang's Wedding Dress in 2009 where I saw my cousin do choreography to it.
Super junior - sorry sorry