For me, I could watch Liar Game and Your Beautiful over and over again
I'd say Nodame Cantabile and My Girl ^^
Coffe prince, Protect the boss, Autumns Concerto......
Iljimae , My Girl , Sweet 18 , The princess'man , My Lucky star , Mars , HanaDan , last friends , Kisarazu cat's eye and all the dramas rated 10 in my dramalist . The movies rated 10 in my list^^
You're Beautiful and Onmyoji (movie).

Looks like Just You is becoming the same, but I won't have a full verdict until it finishes airing.
Arang and the Magistrate, Vampire Prosecutor 1 & 2, any Lee Min Ho drama except Boys Over Flowers, You're Beautiful, and The Master's Sun, even though it's not finished yet...I keep rewatching already-aired episodes while I wait for the new ones.
Startling By Each Step and That Fool
I just finished re-watching Nodame for the nth time!!! I dramathon the 11 episode drama, 2 special and 2 movies within a day and like I'm watching it for the first time. And I still love them. I'm still with dreamy land of Nodame :)
My Boss My Hero, Suzuki Sensei, Jikou Keisatsu, Aku no Kyoten, and Kirishima
These are the ones I can watch over and over again!! Oh! My Lady. Just You. Master's Sun. Down With Love. 9 Ends 2 Outs. Couple or Trouble. Panda & Hedgehog. Snow White. Yamato Nadeshiko Shichi Henge.
Coffee Prince and Lie to me.
City Hunter, You're Beautiful, Answer Me 1997, Shut Up Flower Boy Band, I Hear Your Voice The Master's Sun, Who Are You, and Two Weeks are likely future candidates.
I have never re-watched a j-drama, because there are so many that I haven't watched yet and I'd rather spend my time watching something I've never seen before... But if I were to re-watch something from my still rather short list (29 titles), I'd go for Kimi wa petto first and then maybe Last Cinderella. At some point I also want to re-watch Rich Man Poor Woman, because it was the first j-drama I ever watched and I wonder if I'd like it as much when I've got more to compare it with.
Sungkyunkwan Scandal
Heartstring , boys over flower , kyou koi wa hajimemasu