If just going by economic strength: US + any EU country. As a bonus, if you are of ethnic Chinese descent, you can look into one of the "greater China" regions

If just going by economic strength: US + any EU country. As a bonus, if you are of ethnic Chinese descent, you can look into one of the "greater China" regions of HK/Macau/Taiwan as well, depending on where you/your parents were born. That combo would give you the ability to live and work freely in the three most important economies in the world (US/EU/CN).