Australia and South America, because Australia can be defended in a single territory (Indonesia), against a single territory (Siam) and South America can be defended in two territories (Venezuela and Brazil), each against a single territory (Central America and North Africa respectively.)

In South America, you can get lucky and take North America in 1 or 2 turns. Since North America shares a front with South America, you end up with the same number of fronts you would have if you possessed North America alone, and in a very defensible position. The fewer fronts you have, the more interior you have that doesn't need to be manned with troops.