That depends on what you mean by accepting. Culturally or legally? Legally the immigration process to the US has always been one of the hardest and the bureaucracy around it is a nightmare.
New Zealand and Spain you really couldn't be more wrong. Just for starters, both countries combined accept only ~50k immigrants per year (the US is 20x that).
If you calculate the "expected value" for a migrant from anywhere in the world, the best bet you can make will predict that they'll end up migrating into the US. From this metric, I'd judge that the US is the "easiest" destination.
Btw, I say this as an immigrant in the USA ;)