This was a tough list to compile! Especially since there are dramas that are really addicting the first time around but have no replay value. So here's a list of the dramas that I could watch over and over again without becoming bored :)
1. Sora Kara Furu Ichioku no Hoshi (it's been my favorite for over two years now. I doubt it'll change :D I just love how all the characters are portrayed, and they have a good balance between the heavy, depressing plot line, and the more humourous moments. This is the only Kimutaku drama I like! And I usually avoid his dramas because he mostly portrays the same type of character [the cool, good-looking guy], so that should tell you something about how good this drama is :))
2. Majo no Jouken (Wow! Until this drama I was convinced elementary school kids couldn't act as well as adults. The main girl in this drama won the award for best newcomer as well. Near the end it did start getting a bit iffy because of the complaints received about the harsh punishments the teacher gave out to her students in the drama, but a highly entertaining series nonetheless!)
3. Okane ga Nai (a really OLD drama starring Oda Yuuji, but really, the dramas lately are no comparison to the stuff they used to produce. This series was hilarious, I never laughed as hard as I did watching Okane ga Nai. There are so many memorable scenes---I'd love to own this on DVD but since it's such an old series I have a hard time tracking it down!
4. Ichi Rittoru no Namida - I don't usually watch dramas with one of the leads belonging to a boy group because usually they have no idea how to act (well, with the exception for Dohmoto Tsuyoshi, but he's another story all together). But the guy who portrayed Asou-kun made me (kind of) understand why girls go crazy over boy groups. His acting does have its down moments but at his best, I'm impressed. The highlight of the drama for me was when Aya called senpai to say "goodbye" to him. Just the overal atmosphere and the song playing in the background...definitely a tear-jerker!
5. Akuma no Kiss - Japanese dramas usually stay on the "safe" side dealing with "safe" topics like love so I was surprised when I started watching this series. It dealt with soaplands (prostitution), lesbianism, cults, loan sharks, and drugs. But it doesn't come across as "overdone" because these issues are dealt separately by the three main characters. This is one of the few series that didn't disappoint me at the climax.