For a fair coin, the proportion of heads should be close to $0.5$. The result furthest from $0.5$ provides the strongest evidence the coin is not fair.

$\frac{2}{3} \approx 0.667$, $\frac{3}{4} = 0.75$, $\frac{4}{5} = 0.8$, $\frac{5}{6} \approx 0.833$

$\frac{5}{6}$ is furthest from $0.5$, so J is the strongest evidence.