The most basic hydroxide is

Basic strength of I_A group hydroxides[M-OH]:

$\stackrel{\mathrm{LiOH}<\mathrm{NaOH}<\mathrm{KOH}<\mathrm{RbOH}<\mathrm{CsOH}}{\to }$

As the size of metal increases the ionic nature of M-OH bond increases and hence M-OH ionisation increases
Therefore release of  ${\mathrm{OH}}^{-}$ion increases and hence basic strength increases
Hint: In case of hydroxides the basic strength is the measure of  easy of liberation of ${\mathrm{OH}}^{-}$ ion in aqueous solution.
Basic strength $\propto$easy of release of ${\mathrm{OH}}^{-}$ions