Who's going to get hit hardest?

US is going to face major economic situations.

Considering that Godzilla practically laid ruin to the cities of Hawaii and San Francisco, possibly killing over a million people in the process would mean lawsuits and destruction fees, possibly putting the toll of the destruction at a very high price.

If they are attracted to nuclear arms US would be the country most in danger, however Russia would be a close second not that any scenes have shown Russia, so I guess it's going to be mostly the US with some attacks on Japan.

I highly doubt nuclear weapons would draw in Kaiju. The thing is, they're extremely well-shielded for safety reasons. The pople who work around them don't want to get cancer, after all, so the warheads are surrounded with lead and other heavy metals to prevent leakage. They're better-shielded than most nuclear reactors. In short, regardless of how the kaiju 'sense' the radiation, they won't be able to see it. Not there. Now, nuclear engines? Like what you'd find on a submarine or American aircraft carrier? Those would light up like the Fourth on any sort of geiger counter or biological equivalent.

*Possible spoilers*

Well if i recall there was a part in the trailer with a russian nuclear container that had goo on it. My guess is that the mutos have been drawn to nuclear energy and drained it from various places around the world, and the janjira incident is just part of a bigger conspiracy

Japan is the appetizer, U.S. is the main course, and Russia is dessert.