Agriculture, manufacturing/industry, space exploration, health/medicine advances, etc., the U.S. outshines all others.
We are the undeniable military leader in the world. Not only were we first to posses nuclear arms, we never used this advantage to conquer another nation, instead we defended others, especially Europe, against tyranny.
And the fact that America is the destination of choice amongst most of the world's emigrants lends credence to our being greatest.
As for the attribute "strongest" - who wonders?
Not we.
The US consists of Europeans, Africans and Asians,
The U.S. consist of Americans.
with plenty of rich land even dummies would have made it strong.
So then what's Europe's excuse?
Add the favourable strategic position (with two oceans as natural borders an invasion wasn´t even an option for Hitler).
Germany didn't have to invade us, we went to them, twice.
Eventually, as for the attribute "richest" look up countries like Norway, Switzerland, Monaco or Liechtenstein and feel the way we other Euros feel: poor!
Well then, if another Schicklgruber arises you all can have the wealth-distributors: Norway, Switzerland, Monaco and Liechtenstein, defend you.
Emigrants (at least most) seek prosperity. They choose the country where they can get it with few problems. They come to Europe (from Africa) and to the US (from South America, Middle America). It´s not you, it´s the wealthy countries.
Europe´s excuse? I´d say thousands of years of wars, including dictatorships, oppression up till 1991 (and if we count Belarus, up till today). The US had only one time a war on its soil.
You lack history of Germany - in WW1, no foreign soldier set his foot on German territory, but thanks to the Versailles Treaty, the roots were planted for the rise of nationalism.
So the US consists of Americans? Ok, where did they come from? They´re almost all from other continents. Give me huge land, with the guarantee of few conflicts, and at the same time oppression, famine, dictatorships in the rest of the world and I´ll ensure you that I get the most-skilled people to immigrate.
"Rich" is an adjective that is in no relation with military.