I suppose I should really say New Zealand, as I have family there (my aunt emigrated years ago, and her entire family, children and whatnot, were raised there). However, I'd just tell them to come over here, back to the UK, where they'd be safe. The USA... well, I'd personally enjoy slightly the power reversal, they always boss us Brits around (despite the fact they exist as a country because of us...), so having the Yanks come begging the UK for aid would be kind of enjoyable (I'm a horrible human being, I know, but then again I do have the 'Chilly' badge from that recent quiz...). And as for Japan, well, I do love anime and the whole Japanese culture, but again, they could easily flee to the UK and do all that again.
No, my choice has to be Ireland. I love Ireland so much, I'd love to just spend a few months touring the whole country, and besides, it's the home of Guinness, singer/actress Niamh Perry, and the great comedians Ed Byrne, Dara O'Briain and Patrick Kielty. And the Irish accent is literally like liquid sex...
So yeah. I'd have to save Ireland. No question about it.