Summary: It’s very important to know who the Great Harlot is if we are to obey the command in Rev 18:4, "Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues."
Rev 17:18 And the woman which thou sawest is that great city, which reigneth over the kings of the earth.
Revelation, earlier, tells us which city is THE GREAT CITY: it is Jerusalem.
Rev 11:8 And their dead bodies shall lie in the street of the great city, which spiritually is called Sodom and Egypt, where also our Lord was crucified.
The Lord was crucified in Jerusalem, the great city, which is the great harlot.
Babylon, as a name for Jerusalem, is not unlike the comparison to other cities, such as as Sodom, Egypt, and Gomorrah; "spiritually is called Sodom and Egypt" Rev 11:8. (Isaiah 1:10; Jer. 23:14, Ezek. 16:44-56 all compare Jerusalem to Sodom. (also, Jesus in various NT scriptures, compared various Isralite cities to Sodom and Gomorrah by comparing judgment), Luke 10:12, Mark 6:11, Matthew 11:23-24, Matthew 10:15.)
As the woman sits on seven mountains, Jerusalem is built on seven mountains:
Rev 17:9 And here is the mind which hath wisdom. The seven heads are seven mountains, on which the woman sitteth.
Mt. Goath, Mt. Gareb, Mt. Acra, Mt. Bezetha, Mt. Zion, Mt. Ophel, and Mt. Moriah.
A second source, http://www.angelfire.com/pr/truth/whore.html, lists the same: Mount Zion, Mount Ophel, Mount Moriah, Mount Bezetha, Mount Acra, Mount Gareb, and Mount Goath.
A third confirming source, http://jesus-messiah.com/prophecy/rev-13.html, lists the same seven, and explains a bit more of why some authors list different mountains, and includes a map: "The seven hills identify Jerusalem not Rome: Jerusalem was a large city spreading outside the walls and covering the seven hills there during the time of Jesus and the Apostles: 1.) Mount Gared; 2.) Mount Goath; 3.) Mount Acra; 4.) Mount Bezetha; 5.) Mount Moriah; 6.) Mount Ophel; 7.) Mount Zion. For what ever reason, and by what ever hands and manipulation, Mt. Gared, Mt. Goath, Mt Acra, and Mt. Bezetha have been deleted from nearly all maps of Jerusalem."
http://www.unlimitedglory.org/txtrevcha4.htm "look at any old map showing the topography of Jerusalem immediately prior to 70 AD. In so doing I believe that you will find that the 7 hills of Moriah, Ophel, Zion, Bezetha, Golgotha,
Bareb and Offence are all or partially located within the confines of Jerusalem as it existed at that time."
I could not find Mt. Acra or Mt. Bezetha in scripture, but five others I could. Gareb & Goath in Jeremiah 31:39, Ophel in 2 Chronicles 33:14, Moriah in 2 Chronicles 3:1 and Zion yielded 153 occurances.
Jeremiah 31:39 And the measuring line shall yet go forth over against it upon the hill Gareb, and shall compass about to Goath.
2 Chron 33:14 Now after this he built a wall without the city of David, on the west side of Gihon, in the valley, even to the entering in at the fish gate, and compassed about Ophel, and raised it up a very great height, and put captains of war in all the fenced cities of Judah.
2 Chron 3:1 Then Solomon began to build the house of the LORD at Jerusalem in mount Moriah, where the Lord appeared unto David his father, in the place that David had prepared in the threshingfloor of Ornan the Jebusite.