I love historical ninkyo yakuza (old school) films with Ken Takakura, Koji Tsuruta, Junko Fuji etc. These films were all about honor, loyalty and friendship between men. But the 70's jitsuroku (docu-drama) yakuza films are no doubt more in place on grindhouse forum so I will limit my answer to those (and close relatives).
I'm not the biggest fan of Fukasaku's yakuza films - in fact my favorite Fukasaku movies are dramas and adventure films - but I do love Yakuza Graveyard (1976). Fukasaku's films balance between hectic visual storytelling and substance, and to me often the latter is sacrificed in his hands. But Yakuza Graveyard finds the perfect balance.
That's not to say I dislike his other yakuza movies. On the contrary, I enjoy most of them and just recently purchased all the New Battles Without Honor and Humanity films. I just don't rank them as highly as most people.
Sympathy for the Underdog is really good. I came out between the two yakuza waves - ninkyo and jitsuroku - and has some elements of both, although it's a modern day set film.
Fukasaku is the best known jitsuroku yakuza director, but there were many others, too. Sadao Nakajima for example was a seminal director in the genre. But his films have not been distributed outside Japan.
Same phenomena can be seen with actors. Bunta Sugawara is known almost exclusively as the yakuza film star outside Japan, but actors like Hiroki Matsukata were also remarkable stars in the genre. Most overseas viewers are in fact familiar with Matsukata's face, just not his name. He plays supporting roles in many classics. His leading role movies have not received much international recognizion, unfortunately.
And yes, Sugawara did much more than just Yakuza films. Just check out Norifumi Suzuki's hugely enjoyable drama-comedy-action series Truck Yaroo, where Sugawara does a lot of physical comedy.
One film I want to point out. 60's / 70's master filmmaker Hideo Gosha's (most of his best films remain unrealeased outside Japan and France) Violent Streets (1974) is one of the genre's high points; as violent as Sam Peckinpah, as stylish as Dario Argento, as great as Fukasaku at his best.