There are three main infraclass taxa into which the mammals are divided. These divisions are based on how the young are born. Of the monotremes, the marsupials and the placentals, the placental...
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...
In terms of brain capacity, the most complex mammal is a human. In order to answer your question fully, you need to clarify what you meant by "complex."