Which hydride among the following is less stable?

${BeH}_{2}$ 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 $Be - H$ 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.