I played through the entire fucking series including the PS2 games and below are my top 5 in no particular order.
1. Yakuza 2 (PS2) & Kiwami 2
>They're a package deal. Y2 was the game that really cemented the franchise for me. First game was still great.
2. Judgement
>Lost Judgement can eat a dick. Sure it had better combat, but god damn was the story a letdown. I love how this game got me to engage with Kamurocho to the point where it felt like home moreso than any other title.
3. Yakuza 0
>I hate how much I enjoy this game. It's not perfect by any stretch and has a PLETHORA of issues, but it's still worth playing.
4. Yakuza 4
>Favorite version of Kamurocho. After coming off of Yakuza 3, this game felt next gen.
5. Like a Dragon
>I don't want to recommend this, but truth be told I had a good time. I think the RPG setting really suited Ijincho more than Lost Judgement. And the story was its own thing yet rewarded those that stuck with the entire franchise.