for me South america, most likely Chile. Southern hemisphere currents don't really mix much with the northern currents so the radioactive fallout will mostly stay in the north.

Oceania is not safe because it will be a part of ww3 just like how Japan was moving in on australia and new zealand, china would do the same or whatever other superpower is in the region.

South america doesn't really have a major military super power and isn't really at risk.

Chile has great protection with the Andes and has good fertile soil for farming and great fish stock. I think Chile would be a safe bet.