From what I've learned, Ellis Island was once America's main point of entry for immigrants by sea, the place where people entered the Land of Opportunity.
I imagine there's an airport with a similar reputation, but given planes go pretty much everywhere these days, immigrant entry probably happens at a number of airports. Which airport has that reputation?
When I went on a school trip to America, we landed at Newark Liberty for initial processing before we took a domestic flight to our first destination airport. Perhaps that could be it..
EDIT: UNDER IDEAL CIRCUMSTANCES! Assume immigrant-neutral government policy.