Who created the largest colonial empire?

Great Britain, by far. At its height, the British empire covered almost 37 million km2 and held sway over nearly 460 million people in the early 20th century. The Spanish Empire at its height in the 18th century was around 20 million km2, and the French colonial empire reached an area of 12.5 million km2. As you can see, the British empire was the largest colonial empire in history, and in fact, the largest empire of any kind in the history of the world. The only other empire that came close to being as large as the British Empire was actually the Mongol empire, which reached an area of 33.2 million km2 by the mid 13th century.