What is the principle that the simplest explanation is usually the best?

The principle that the simplest explanation is usually the best is known as Occam's razor.

Occam's razor is a rule of parsimony: when competing explanations account for the same evidence equally well, prefer the one that requires fewer unnecessary assumptions. It is associated with William of Ockham, a 14th-century English philosopher and theologian, although the underlying idea appeared in the writings of earlier thinkers, including Aristotle and Thomas Aquinas.

The razor is often summarized as “the simplest explanation is usually the correct one,” but that wording can be misleading. It does not mean that the simplest idea is always true, or that scientists should ignore complicated evidence. A more accurate use is to choose the simplest explanation that fits the available facts and makes testable predictions. New evidence can show that a more complex theory is necessary.

The tempting alternatives are different principles. Occam's broom refers to ignoring inconvenient facts, Hitchens's razor concerns claims that lack evidence, and Hanlon's razor advises against assuming malice when incompetence is sufficient. None is the principle described in the question.