Location (which includes physical isolation from possible attackers), large population, natural resources (including relatively mild climate in large parts of its territory), available space for building, stable and relatively quick justice system, top notch advanced education, enormous scientific and industrial base. It's not as much as the US should or must be #1, it's just that it's hard not to be #1 given all of this.
China + Russian are also a great combination but have higher barriers (tougher climate, more variegated cultures and languages, separate political system, etc.)than USA.
Most of Russia's territory has hostile, cold weather. It has a ton of resources but also a ton of problems which explains its low population for the size of the territory. China is a closer comparison and no wonder it grew so quickly in the past decades.
