The most acidic and thermally stable hydride of chalcogens are respectively

H₂Te; H₂O

On moving down the group, the size of the atoms \ions of the elements in that particular group gets increased. In the oxygen family, the size of the ions follows the order:

O⁻²< S⁻²< Se⁻²< Te⁻²

Hydrogen will be more strongly bonded with O₂⁻ and least strongly with Te₂⁻ because of larger size of  Te²⁻. So, it is very easy to remove H⁺ ions from H₂Te.

Hence, H₂O is thermally highly stable and H₂Te is the most acidic hydride.