Which is the broadest classification level?

Taxonomists use "domain" to denote the highest possible classification of organisms. Above the level of the domain, the only group organisms belong to is life itself.

The two recognized domains are Prokaryota and Eukaryota. The plant, animal, fungal and protist kingdoms are eukaryotic, which means that they have large body cells with specialized organelles. Archaea and Eubacteria are kingdoms of Prokaryotes. They are all single-celled organisms that live in nearly every environment on Earth. While some Eukaryotes are also single-celled organisms, such as the amoeba, they are more closely related to humans than either amoebae or humans are to bacteria.