What is the oldest group of placental mammals?

This was incorrectly presumed, by 19th century scientists, to be platypus. It is actually uniquely evolved.

If you're attempting to find out the most primitive "modern" mammals, they could be argued to be the Soricidae, the shrews and the Talpidae, the moles, but any modern creature is simply not primitive. More basic, more complex, more or less evolutionary divergent from their ancestral norms, but not more primitive.