We're number 1 in history for lots of reasons. But 3 are so big as to make the others irrelevant.
#1. Geography: no near peers on our continent after the 1800s, lots of natural resources.
#2. Freedom: free political systems lead to more efficient trade and more wealth. Historically the wealthiest countries have the best armies.
#3. Until relatively recently we picked our wars well. We only fought when we were driven by strategic national interest, and protection of economic strength and national ideals that a strong majority would support for the duration of the war.

Our lead in aircraft carriers and aircraft are significant. Our Navy's capital ship is the aircraft carriers while China's and N Korea's capital ships are the submarines.

Our funding really sets us apart. We would rather waste money than lives. We are one of the few militaries that really adhere to this. We invest in system that give us a clear lop sided victory. You can have all the tanks you want. Russia is an example of this. But when we send out fleets of attack drones and then attack aviation behind that their tanks would even be able to fire on our tanks. By the time our tanks arrive there really won't be much that can stop them. This is the same with our navy. We can project so much power that by the time a carrier group arrives the battle is pretty much over. Our drones are redefining the way we fight wars. Soon we will have attack drones that will attack other drones. Yet again. We will own the sky.

On top of that we can radically implement new weapon systems. We found IEDs were a threat to us. So MRAPs were made. We made so many we were just giving them away. I don't know another military that can do that on our scale. We can answer a situation like no other. By the time a country like Russia or Iran figures out how to counter something we are moving on to a different strategy.