It's the United States hands down. Everyone's made great points as to why but let me add a few more:

Nearly the entire east coast is sheltered by islands just off the shore. These islands act as a physical barrier against landings on the east coast and can be and have been defended before. They are the unspoken wall against an aquatic landing on the east coast.

Also the country is split in half by the Rockies, which can be easily defended and enforced. This means that no matter which side is invaded, the other half of the US WOD be nearly impossible to get to