An understatement for the purpose of emphasis is known as

Litotes is a figure of speech in which an affirmative idea is expressed by negating its opposite, producing a deliberate understatement for the purpose of emphasis. For example, saying "not bad" to mean "good," or "she is not unkind" to mean "she is kind." By softening the expression, litotes paradoxically draws more attention to the quality being described.

The other options describe different literary concepts. A panegyric is a public speech or written text that lavishes praise on a person, event, or achievement. Hyperbole is the opposite technique: it uses exaggeration rather than understatement to create emphasis (e.g., "I've told you a million times"). Climax in rhetoric refers to the arrangement of ideas in ascending order of importance or intensity, building toward the most significant point.

The defining feature of litotes is that it understates something in order to make the point more forcefully, which matches the description in the question exactly.