I think I'd have to say Canada. I'm not actually American or Canadian, I'm from England but I have a lot of friends from all over the world. The question 'who is the most like who' is something that does come up a surprising amount, and what we've agreed on is that the people closest to you are the ones most like you. For Americans that would Canadians, for the English it's the Welsh and the Scots, and for Australians it's Kiwis. For people who come from places which are further away, my friends and I have also pretty much agreed that the English and the Australians are rather more similar than what the Americans are. I think that probably has something to do with the way we use language? I read an article recently which explained how Americans tend to be overstate everything (that was an AMAZING meal), which makes them sound fake to us Brits, and the British tend to understate everything (meh, it was okay,) which makes us sound cold to Americans.