Practically speaking, I'd have to say Canada because of the same thing that keeps us where we're at right now - the desire to be close to family while our kids are small. Canada is 5 minutes away, so that's an acceptable distance. No other country would be. I have no desire or intention to move at all until the kids are grown or nearly so.
Take away the practical worries and I'd have to say New Zealand. It is such a beautiful country with such a fascinating culture, a pleasant climate, and social norms that are rather more in line with our priorities than the American way of life. Would I rather live there? If it were just me, yes, but my personal preferences aren't reason enough for me to move my kids a world away from the grandparents, aunts/uncles, cousins, etc. that they have here.