French Revolution
From Tardis Wiki, the free Doctor Who reference
The French Revolution was a series of political and social upheavals which occurred in France between 1789 and 1799. This was the Doctor's favorite period of Earth history.
Revolutionary-era France was the Doctor and Susan Foreman's first-ever visit to Earth. (NA: Just War)
While attending Coal Hill School, Susan borrowed a book about the French Revolution, but noted that it had incorrect information on the first page. (DW: An Unearthly Child)
The Doctor and Susan visited Revolutionary-era France again, this time with Ian Chesterton and Barbara Wright accompanying them. (DW: The Reign of Terror)