The strength of a base is determined by its ability to donate hydroxide ions (OH-) in water. The stronger the base, the more hydroxide ions it can donate.

In aqueous solution, hydroxide ion is the strongest base because it is the conjugate base of water, which is a very weak acid. This means that hydroxide ion is able to completely deprotonate water, forming hydrogen ions (H+) and hydroxide ions.

The strength of a base can also be expressed in terms of its pKb value, which is the negative logarithm of its base dissociation constant (Kb). The lower the pKb value, the stronger the base.

The pKb of hydroxide ion is 0, which means that it is a very strong base. This is because hydroxide ion is completely dissociated in water, meaning that there is no equilibrium between hydroxide ion and water molecules.

Other strong bases in aqueous solution include:

* Sodium hydroxide (NaOH)

* Potassium hydroxide (KOH)

* Calcium hydroxide (Ca(OH)2)

* Barium hydroxide (Ba(OH)2)

These bases are all strong because they are all able to completely dissociate in water, forming hydroxide ions and the corresponding cations.