The most soluble hydroxide among the following is

Solubility : Be(OH)2 < Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2
As lattice energy decreases solubility increases
If anion size is small F^-,OH^- ,O^2-then with small change in cation size , then there is a considerable change in lattice energy. And hence in this case the solubility depends upon the lattice energy
In a series of compounds with increases in the size of oppositely charges ion lattice energy decreases there by solubility increases