In the taxonomic ranking of animals (Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species) what is the highest rank that has only a single species in it? An example is the tuatara which is the only species in the Order Rhynchocephalia. Are there any species that are an only example of their Class or above?

By the taxonomic ranking of animals, the highest monotypic animal (single species) rank is the Phylum of Placozoa.

Placozoa, meaning 'flat animals', are groups of cells that live in water and look like miniscule, shape-shifting disks.They are described as 'the simplest animals on Earth'. https://en.wikipedia.org/wiki/Placozoa

It is believed that there may actually be more placozoan species existing, however they are yet to be identified and classified. The extra placozoan species may well turn out to occupy several genera and families and perhaps even a few orders. https://www.sciencedirect.com/science/article/pii/S0960982217315245