I'm an American and I'm very fond of the United States. But I'm not sure that I'd go so far as to say that it's the "best country in the world". Different people value different things and will probably favor different countries, cultures or geographies depending on their taste and mood.
Here are my reasons. Feel free to contribute.
1: We're number 1 in most sports; baseball, basketball, football, soccer, running*other Olympic types of sport*.
We're typically the best in sports that are big in the US. We aren't so hot at sports that aren't all that popular here. American football - big. Soccer football - not so much, for men at least. Cricket and rugby? Just for weirdos here, but huge some other places.
2: We have the best, most influential economy. America has a crisis, the rest of the world has a crisis.
The European Union has as big an economy as the US in aggregate. China's on course to pass us. Of course, China has something like 4x our population, so we would still be way ahead per-capita. But there are smaller countries that are richer than us, person for person.
3: We have the most rights/freedom.
Maybe. Some of the Europeans, the Australians, Canadians and so on do pretty well too. We're near the top, but not necessarily at the top, for most kinds of freedom I guess. It varies by what kind of freedom we are talking about.
4: Our government is proven to be the most caring for its people.
I don't agree with that one. There are quite a few governments out there that are more paternal and run bigger welfare-states.
5: We have the best music. The Beatles, Jimi Hendrix, SlipKnot, Queen, The Rolling Stones, Lil Wayne*And all the other best rap/r&m artists*, Michael Jackson, etc.
Maybe so, if your thing is contemporary Western popular music. I'm not particularly interested in that. (I don't consider "rap" to even BE music.) Europe's produced the best classical music. The Africans have some amazing musical traditions, as do the many Asian cultures. It's all a matter of taste.
6: Most powerful/successful army in history.
Since World War II perhaps. The British were equally successful for a couple of hundred years before that. The Spanish used to be very big. And there's that old classical favorite, the Romans.
Check out India some time.
And there's the European Union. 27 countries, kind of on the land-area scale of American states, though typically with bigger populations. Just about all of them has its own national history and its own language and literature.
8: Best movies in the world.
We do seem to be the center of the world's film industry at present.
9: Best scientists/ most modern inventions and breakthroughs.
That's been true since World War II. The Europeans effectively lobotomized themselves in the war, the Chinese did the same thing with their 'cultural revolution", kind of leaving the US the leader by default. But the Europeans seem to be recovering their scientific energies and are winning a larger percentage of Nobel prizes again these days. Lots of scientific work seems to be coming from there. East Asia has potential as a science and engineering center too. There's stirrings in India and there's always Russia. Brazil and Argentina might make some noise later in the century.
The US will probably remain among the world scientific leaders in the 21'st century, but less clearly in the number one spot. What we are probably going to see is different regions moving into the lead in different research areas.