I bet almost all of you think it's Arthropoda since there are so many insects and especially beetles, which God was very fond of.1
However, recent advances in taxonomy have suggested that there are one million species of nematode. This is about the same number of species as arthropods so the primacy of insects has been challenged.
1. This is a reference to a famous quip by J.B.S. Haldane. When asked to name the most important thing he has learned about God from studying biology he reportedy said, "I'm not sure, but He seems to be inordinately fond of beetles."
[Photo Credit: Iowa State University Plant Disease]