Ouroboros, Oguri shun is a small time yakuza boss
Crows zero Oguri Shun play yakuza bosses son

Gokusen very good as well

Yankii-kun to Megani chan high school romance type but gangster as well

cambrian period is one of my favs 

Kid Gang - a comedy K-drama from 2007 that follows three gang members left to care for their leader's baby. It focuses the characters brotherhood and all the crazy things that they get into.