China and it isn't even close.
They hold a ton of US debt.
They are basically economically colonizing the world. From building islands to extend their territory in the South China Seas, to building infrastructure in the 3rd World, Africa, South America, etc. They will be unstoppable. They will OWN these territories.
I also believe they will eventually commit mass genocide or other forms of population control.
They are doing this slowly and quietly. They are very patient and they are ruthless.