Theres so many good ones ^.^
1 Litre of Tears
Hana Kimi
Hana Yori Dango
Kingyo Club
Liar Game
Life
Taiyo No Uta
W no Higeki
Yamato Nadeshiko Shichi Henge
I'm pretty new to the Jdrama scene, but I don't think it's going to be easy for me to find something I like more than my current favorites.
So, my favorite Jdramas is actually a tie between Arakawa Under the Bridge and Majisuka Gakuen 2. They are quite different but still they share the same type of eccentric anime like characters that are colorfull in both personality and appearance. Every episode is a blast and I had to restrain myself from not watching these series too fast so that they would last longer.
Not in order, but:
Hana Kimi 2007
Nodame Cantabile
Nobuta wo Produce
Gokusen 1, 2, 3, the Movie
Ouran High School Host Club
One Pound Gospel
Yamato Nadeshiko Shichi Henge
Tumbling
Atashinchi no Danshi
Yokai Ningen Bem
Can't decide on THE most favorite drama, there are quite a few:
Yasha
Nobuta wo Produce
Hana Kimi
Hana Yori Dango
Nodame Cantabile
Kurosagi
Orthros no Inu
Code Blue 1 + 2
Rookies