Yamato Nadeshiko Shichi Henge was the first one I finished, but before I got sucked into Kdramas, I found Zettai Kareshi on Youtube and watched a few episodes.
I honestly don't remember which Jdrama was my first. I think I watched a bunch of Jdramas in a row.
If I had to make a lucky guess, then it was probably "Love Revolution". And I really loved it.