There is no definitive answer to this question as it largely depends on how you define "EdTech company." For example, if you consider a company like Google to be an EdTech company, then it would certainly be the biggest EdTech company in the world. However, if you only consider companies that specialize in educational technology, then there are a few candidates that could be considered the biggest EdTech company.
Some of the leading EdTech companies in the world include Blackboard, edX, and Udemy. Blackboard is a company that specializes in providing software and services to education institutions. edX is a non-profit organization that provides online courses from leading universities. Udemy is a company that provides online courses from a variety of instructors.
So, which is the biggest EdTech company in the world? It really depends on how you define "EdTech company."