What ion of barium is most stable?

So upon oxidation it commonly forms a ${\text{Ba}}^{2 +}$ ion:
${\text{Ba" rarr "Ba"^(2+) + 2"e}}^{-}$
So how do you know this? Well, you will always have access to a Periodic Table, and the $\text{Group Number}$, the vertical column in which the metal lies is $\text{Group 2}$. The $\text{2}$ tells me there are 2 valence electrons, whose loss gives the ${\text{Ba}}^{2 +}$ ion.