Which hydride among the following is less stable?

${\mathrm{BeH}}_2$ is indeed considered to be less stable compared to other hydrides among the given due to its significant covalent character and electron deficiency. Beryllium, being a small and highly charged cation, attracts the electrons in the $\mathrm{Be}-\mathrm{H}$ bonds more strongly, resulting in a greater degree of electron sharing between beryllium and hydrogen. This covalent character is in contrast to other alkaline earth metal hydrides, which tend to have more ionic character.