Dae Gang Geum aka Jewel in the Palace - after this one there was no looking back! Started with mainly historical dramas like Dong Yi and Yi San then began watching more modern gems.
From what I remember, my very first korean drama was... Full House ! I think I watched japanese dramas before but I don't remember the first one. Probably a manga adaptation.
My first drama in the fall of 2012 was Padam Padam and while I liked it I don't consider it my gateway drama. My gateway drama was Lovers. I watched it late spring 2013 at a time when I was about ready to abandon Asian dramas for good. But I ended up loving this drama so much that I decided to stay and I'm still here today.