All time - have to say London, UK. Birmingham comes pretty close. I love them for all the amazing memories, people and just the feeling of belonging, being somewhere you call home which has never happened with eg. Helsinki where I was born.
I also love Portsmouth, UK again for all the lovely memories. I also really love the south of England.
And with all these places in UK, I guess I could mention New York as well since it was pretty incredible place as well. I still don't know how to get anywhere around there (the street plan is just really confusing me) but oh, oh all the shops and tall buildings that cast shadows on the streets and museums and the weird, lovely feeling you get seeing the Statue of Liberty and the city lights that never go out and you just can't get the same thing anywhere else in the world. Would love to go back there, maybe as a student next year but I doubt that it will ever feel like home the same way England does.