Hard Boiled and War are at the top of the list. The Yakuza Papers is a good series, the first movie is by far the best and they are based on the events of the different clans written by Mino, which his clan Mino-gumi is named the Hirono family in the movies. This series is one of the most brutal series out there. Sonatine is another good one. There are many especially from the 1970's.

I recommend A Better Tomorrow and Internal Affairs for the Triads.