Sadako vs Kayako, who won?

Sadako didn't win against Kayako, and it was a draw.

Finally, Zhen Zi and Kayako merged to form a more powerful monster. Zhen Zi and Kayako have a lot in common, so they formed a strong resentment after their death. As long as they are infected with this resentment, they can't get away.

They are not ordinary ghosts, Kayako is formed by people's spells, and Zhen Zi is condensed by people's resentment, so they are all ghosts with super powers, not ordinary species, so no one can win, and the final outcome is the combination of both sides.

If the theory of one-on-one hit, Sadako is worse, Sadako killed Kayako just by thinking. Kayako came back to life, so Sadako could not destroy Kayako no matter what skill she used.