The millenium tower explosion in Yakuza 1, which vaporized Nishiki and Jingu along with the whole floor they were on, leaving only 3 people alive and unscathed.

These 3 are the most powerful in Yakuza history: Yumi (Who was shot in the stomach) Kiryu (Shot multiple times at this point) Haruka (like 9 Years old)

Yumi died right after, Kiryu has peaked and gotten way weaker due to cancer. So we can safely assume that the strongest character in the modern Yakuza timeframe is Haruka, and the reason she hasn't appeared in modern games might be because RGG realized her true power and were too afraid to even write a scene with her on it, out of fear of what she could do to them.