US - obvious choice
China - second most powerful country in the world, also obvious
India - not a great economy, but still, at this point you'd have like half the world's population. In a total war situation the amount of people you could draft and the amount you could produce from these three alone would be insane.
Surprisingly, the country with the fourth largest army in the world is NK. But I'll leave them off, because while they have the ability to crush most nations right now, they're suffering to support it, and they don't have much potential for growth.
Russia - Energy, fifth largest military in the world, one of the largest populations. Since Putin has come into office they've actually done surprisingly well economically.
Saudi Arabia - lot's of oil to tie it together. Already have insanely strong military might by now anyway, and seeking more would be supreflous. If you just really wanted more military, you could throw in France or the UK. But good luck finding enough energy for your conquest. In such a situation, your first goal would be to conquer Saudi Arabia or Venezuela anyway.
Russia, Italy, and China can easily build a land route to Saudi Arabia, and pretty soon they will have Asia. The US can probably take on all of the Americas by itself, defend oil shipping routes between it and Saudi Arabia, and blockade other important nations. After they're done they can probably just tell Europe to give up. Africa isn't even worth bothering.
Japan and Germany have good economies, but ever since they lost the war they've been pretty much ignoring their military. The eurozone itself is extremely strong collectively, but there are just better choices than any one of them individually. If I were allowed to pick the EU, it would be an obvious second choice, and I'd bump Russia off the list. An alliance like that wouldn't even be fair - it'd be all the strongest nations in the world besides Japan, Russia, and Canada, and it'd represent like 75% of the world's GDP and almost all of its military power.
