This is an overview of the World Top 3 Oilfield Service Companies based on 2019 revenue and current market capitalisation.
Schlumberger has always maintained its position as the world largest Oilfield Service Company. Today, Schlumberger is still the world largest Oilfield Service Company based on Revenue and Market Capitalisation.

Halliburton used to be the second largest Oilfield Service Company, but is not the case anymore. Baker Hughes is taking over as the second largest Oilfield Service Company. Halliburton is now rank number 3.

It would be interesting to see if Baker Hughes -which is currently promoting itself as an energy technology company rather than an Oilfield Service Company- could dethrone Schlumberger and become the world biggest Oilfield Service Company.