It’s Rome:

The New Testament A Historical Introduction to the Early Christian Writings, Oxford University Press The beast has seven heads, and we are told that these are seven mountains on which the woman is seated (v. 9). For those who know enough about the world in which the prophet was writing, this will be the only clue that is needed. For those who don’t, the angel makes the matter still clearer in verse 18: “The woman you saw is the great city that rules over the kings of the earth.” The meaning of the vision is now reasonably transparent. The “great city” that ruled the world in John’s (Author of Revelation) day was obviously Rome, commonly called the city “built on seven hills” (hence the beast’s seven heads). This city, which in the vision is supported by the Devil himself, had corrupted the nations (the whore fornicates with the kings of earth), exploited the peoples of earth (she is bedecked in fine clothing and jewelry), and persecuted the Christians (she is drunk with the blood of the martyrs).