The country that held the most land was Britain
Step-by-step explanation:
Britain had been the most successful country in terms of the land that they were able to colonize during the time of the colonization.
This means that they held over a quarter of all the land of the world and nearly one third of the population under their control in the peak of their powers.
This is something that no other modern empire had ever achieved in its time and made Britain the most successful country in term of colonizing in the world by far.