Hmmm going to have to say China, not because they are my favorite country but because I simply see them as the safest and smartest choice.
Their economy is still growing rapidly and the country is developing more each day, they still have massive widespread problems but the central control system can have over a million soldiers working on any project in the country in under a week.
They don't have or get financial problems. THe American system, and the entire country is built on debt and credit, this is a atrocious system as as been seen many times in the past 100 years. The Chinese system is built around only spending what you have and lending out at good rates to turn what you have into more. This is why America owes them so much that if half of its GDP was diverted to purely paying off debts they would be doing so for many many decades.
Massive stores of natural resources that can be developed including a huge proportion of the rare-earth metals currently on market.
Size and numbers, they are huge and populated.
One of the leaders in Thorium Nuclear power, Cleaner, safer and better power generation.
Basically they are easily in the best position in the world to take it over or win any conflict. I know many of you will immediately quote 'Team America' at me but the facts are kinda clear. America or any of it's Allies threaten china, china refuses to lend America money the next year but still collects on it debts. America can't actually pay for it's own military so is forced to scrounge loans off the rest of the world and looses some of its credit rating. Within 2 years it's rating is so bad that it can't get good loans and plunges into a recession worse than anything before. Of course it could default on it's loans to China and refuse to pay but again its credit rating dives and without china paying half its bills every year its credit plunges and recession hits, can't upkeep it's own military. America simply isn't a threat to china for these reasons.
America cannot function without massive loans EVERY SINGLE YEAR from China, they pay half its bills and military costs.
Lets say China moves first and announces that it will no longer be exporting rare earth metals, oil/gas or lending money. The western tech markets will crash as it will take years to increase rare earth production enough to meet even current demand. Half the western economies will crash and hit recession within 2 years, lead by America who instantly would begin defaulting on loans and couldn't pay their own military or goverment. Many researchers predict an American civil war within 5 years (I can picture the confederate flags marching again)
Even if America threatened Nuclear war or sanctions, then China can reply "Mutually Assured Destruction" and "We are Solvent" China has no debt and produces everything it needs so sanctions wont hurt it while most of the Western world would once more hit recession.
With the Western world so deep in recession that they are turning to civil war, China can easily expand forcefully in asia as Russia wouldn't be fairing much better, until it was basically in charge of most of asia and encroaching on Russian territory.