London

It's probably a bit of a mystery as to precisely why London in particular seems to draw so much attention from the strange and alien; lacking even the Rift that Cardiff possesses, it nonetheless seems to excel in harboring and attracting innumerable hostile races and beings. This also means that it remains one of the cities most often visited by the Doctor, a habit that has admittedly fluctuated over the last few decades of the Time Lord's life; though he doesn't visit quite as often as he did in his Ninth or Tenth incarnations, he might pop by for a day or two... make sure nobody's trying to burn it down again.