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Sources conflict each other, but there are three possible answers: Rassilon, Omega, and the Other. Some sources say they worked together.

The most common answer is Rassilon. He was the lord president for millennia, and is said to have created the first TARDIS. ("The Deadly Assassin", "The Invasion of Time")

The most common other answer is Omega ("The Three Doctors", "Arc of Infinity".

Yet another answer is that they worked together, along with another person called "the Other," who may be the Doctor. (Virgin Novels, Lungbarrow)