Tough question, as there is overlap between them. To be specific to the question of most useful, I would say conflict theory.
I understand Marx missed a lot of nuance in his criticisms, which feminists and other parties made up for in the future. However, the class conflict described by Marx directly affects our material conditions, and more specifically, flatly spells out issues with the overarching socio-economic super-structure that is Capitalism. At the risk of sounding like a class reductionist, I ask, what is the point of dissecting the nuance of intersectional issues like race or gender before ironing out the insane gaps between standards of living? For example, the wage gap between women and men in North America, although largely caused by systemic sexist policy, is nothing compared to the Wage gap between North America and the Global South, with the latter directly tied to class (proletariat vs bourgeoise) conflict.
With that said, I cannot understate the contributions of the latter theories that built upon Marx’s. I certainly agree that issues such as race and gender intersect with class conflict, but I still see the pinnacle ideas of conflict theory as a solid basis, and with that, most useful
One thing that surprised me was that the text does not mention Marx’s labour theory of value, yet says values, theory, and research are the building blocks of sociology. I feel like this would be an important addition to the little Marx section