What is the simplest purest form of matter

The answer to the question is element

The most chemically simple form of pure matter is that of a pure element, not chemically bonded in any way.

An Element is the simplest form of matter which can not be changed further by chemical or physical methods. An atom is the simplest form of an element. The simplest form of some elements is a molecules. When an element consists of two identical atoms it is called a diatomic molecule.