てしかるべきだ

Expresses that an action or state is only proper, fitting, or naturally expected under the circumstances. The speaker judges that it ought to happen as a matter of course.

A stiff expression found mainly in writing, formal speech, editorials, and arguments about responsibility, rules, or fair treatment. It presents an action as the naturally appropriate outcome rather than as a direct command. It is often followed by もの to state a general norm.

• べきだN2「べきだ」attaches to the dictionary form of verbs, is neutral in register but fairly strong in tone, and can be used to directly assert something or to persuade; it also has the form 「べきだった」 for expressing regret. When commenting that something is as it ought to be, the two are often interchangeable, though 「てしかるべきだ」 takes the て-form and sounds more formal and tactful.
• て当然だN2「て当然だ」focuses on concluding that a result or reaction is only natural given the cause, and it works for both good and bad outcomes (e.g., 「体を壊して当然だ」) without necessarily implying criticism; it is neutral in register and can also attach to adjectives and negative forms. The two are interchangeable only when evaluating that something is as it ought to be.
• のももっともだN2「のももっともだ」attaches to the plain form plus 「の」, is neutral in register, and says that a certain emotion, reaction, or course of action is reasonable and understandable; 「てしかるべきだ」 takes the て-form, is more formal, asserts that something is as it ought to be, and often carries criticism. The two cannot be interchanged.
• ものだN2Only the admonishing sense of 「ものだ」 is close: it takes the dictionary form or the ない form, appeals to social common sense, and carries a preachy tone while remaining neutral in register. 「てしかるべきだ」 takes the て-form, is more formal, and comments on other people or society with implied dissatisfaction, so the two should not be swapped directly.
• なければいけない / なければならないN4「なければならない」expresses objective necessity or obligation, is neutral in register, can be used about your own affairs, and also attaches to adjectives and nouns. 「てしかるべきだ」attaches only to the verb て-form, is more formal, and comments that others ought to be a certain way, often with an implied sense of dissatisfaction.

Compared with べきだ and なければならない, which state obligation or necessity more directly, てしかるべきだ frames the action as the fitting, expected outcome of the situation, close to て当然だ but more formal. のももっともだ instead acknowledges that something is understandable, and ものだ states a general norm or truth without the sense of what the situation specifically calls for.