Barcelona

4a8c0871df799-doctor_who_david_tennant.jpg" Where was I? Oh yeah, Barcelona!" Tenth Doctor, Parting of the Ways
Yes, you're probably thinking, what has Barcelona got to do with Doctor Who, but, it did pop up unexpectedly in one of the 2005 episodes just after Christopher Eccleston had regenerated into the Tenth Doctor, David Tennant and the first of David Tennant's lines was: "Hello... ooh, new voice! Uh, where was I, oh yeah, Barcelona!" And that was one of the most memorable Tenth Doctor lines! This line was to remind us that it was the same Doctor still inside him and it helped Rose and the viewers with the new face as he picked up where he had left off, as just before he regenerated, the Ninth Doctor told Rose about a planet called Barcelona where part of the inhabitants are dogs with no noses.

Another mention of the city of Barcelona was in the 2008 story, The Fires of Pompeii, where Donna says something embarrassing for the Doctor and he says, "You must excuse my friend, she's from... Barcelona." This seems a particular favourite of the Tenth Doctor's!