The best country in the world myth is used as a political tool to make people think all is well and you're better here than anywhere else in the world. USA has this exceptionalism culture in politics and us brits have always had it too. Plenty of others use it.
Are they bad countries to live in? Certainly not. Best in the world? No metric shows that.
Same with the 'land of opportunity'. A myth perpetuated to make people work hard. Reality remains that the most surefire way of being successful is having a rich father.
I'm not saying the US is a bad place because it isn't...I think there have been a lot more people coming to Canada from the US than there is going from Canada to the US at least since 2016 lol...as for the best country I might pick one of the Scandinavian countries but like you I haven't been to that many places to be certain...
I don't know about many other country's, but this one has all I could want, and more, don't know where this sentiment that the US is a bad place to live is, kind of makes me curious, I guess if you go to a good city, I don't see how it gets much better, if you go to a bad city, I'm pretty sure it's on the other end of the spectrum as far as a good city, if all these people come from South America, there is Mexico, Canada, Asia, Russia, Middle East, it's not like America is the only option, therefore it must be somewhere at the top, though not living in many other country's, I can't say one way or another.