Yakuza 0 is an absolute must-play and is also perfect as an introduction to the pre-LaD era. Beyond that you can do Kiwami (remake of 1 tweaked to be more of a sequel to 0) -> Kiwami 2 (likewise a remake of the second game) and then if you're really in love you can try 3-6. The best games (IMO) are 0, 5, Judgment and LaD
Judgment is a(n extremely good) detective spin-off completely isolated from the other games save for the setting and some slight nods to stuff that's been happening in the world. It also has its own sequel, Lost Judgment. You can play these games pretty much anywhere - Lost Judgment does indirectly spoil a pretty big plot development from Like a Dragon, but you don't need to worry about that! Ishin is also a spin-off that's basically a samurai movie with all the characters from the other games "playing" the characters in the story. LaD Gaiden is, well, a gaiden side-story bridging the events of Yakuza 6 and the upcoming LaD sequel.
All the games besides LaD are beat-em-ups at their core but between all the mini-games and other gameplay segments have spanned just about every genre at this point.
LaD is exceptionally long. The average Yakuza game is about half its length with the notable exception of 5, which is a ridiculously chunky game.
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Let the voice of love take you higher, With this gathering power, go beyond even time!