In methanol, toward $\mathrm{CH}{}_3\mathrm{I}$, $\mathrm{CH}{}_3\mathrm{S}{}^{-}$ is a better nucleophile than $\mathrm{CH}{}_3\mathrm{O}{}^{-}$ although $\mathrm{CH}{}_3\mathrm{SH}$ is more acidic than $\mathrm{CH}{}_3\mathrm{OH}$. The reason is that

1. Asulfur is large and polarisable and is much less strongly hydrogen bonded by the solvent than oxygen
2. B$\mathrm{CH}{}_3\mathrm{S}{}^{-}$ is a stronger base than $\mathrm{CH}{}_3\mathrm{O}{}^{-}$
3. C$\mathrm{CH}{}_3\mathrm{O}{}^{-}$ is much larger than $\mathrm{CH}{}_3\mathrm{S}{}^{-}$
4. Dsulfur is more electronegative than oxygen