A current study favors the sponges:
From the 19th century to about ten years ago, there was general agreement that our most distant relatives are sponges. Sponges are so different from most animals that they were originally classified as members of the algae. However, genes and other features of modern sponges, such as the fact that they produce sperm cells, show that they certainly are animals. Their distinctness and simplicity certainly fit with the idea that the sponges came first.
But over the past decade, this model has been challenged by a number of studies comparing DNA from different animals. The alternative candidates for our most distant animal relatives are the comb jellies: beautiful, transparent, globe-shaped animals named after the shimmering comb-rows of cilia they beat to propel themselves through the water.
Max Telford and Pascalia Kapli, “Is our most distant animal relative a sponge or a comb jelly? Our study provides an answer” at The Conversation
The chances are that the genetic analyses suggesting that comb jellies came first may in fact suffer from not accounting for the bias that makes these animals look older than they really are. In the end, our work suggests that the sponges really are our most distant animal relatives.
Max Telford and Pascalia Kapli, “Is our most distant animal relative a sponge or a comb jelly? Our study provides an answer” at The Conversation
The next study will probably say something different.