  The conversion efficiency from charge current to spin current via spin Hall
effect is evaluated by the spin Hall ratio (SHR). Through state-of-the-art
$ab~initio$ calculations involving both charge conductivity and spin Hall
conductivity, we report the SHRs of the III-V monolayer family, revealing an
ultrahigh ratio of 0.58 in the hole-doped GaAs monolayer. In order to find more
promising 2D materials, a descriptor for high SHR is proposed and applied to a
high-throughput database, which provides the fully-relativistic band structures
and Wannier Hamiltonians of 216 exfoliable monolayer semiconductors and has
been released to the community. Among potential candidates for high SHR, the
MXene monolayer Sc$_2$CCl$_2$ is identified with the proposed descriptor and
confirmed by computation, demonstrating the descriptor validity for high SHR
materials discovery.
