The Nordic countries are probably the best places in the world to be average or below, but I'm willing to claim that the US is better for the top 10%.
If you look at income, wealth, employment, education, healthcare, life expectancy, infant mortality, there's a very large difference in the US between the 25% line and the median.
If you don't think you can get into that top 25% bracket in the US, pick another country if you can like Canada, Sweden, Norway, Finland, Denmark, Netherlands, Australia, New Zealand, Germany, Austria, etc.
40%-45% of the world's millionaires reside in that top 25%. Full-time employment will get you $80,000+ at the 25% bracket cut-off. Healthcare is broadly high quality and easily accessible for the top 25%. Economic opportunities are abundant and unemployment is always very low outside of deep recessions. You can also hit that $80,000 income line in dozens of cities, depending on profession (much easier in tech, energy, healthcare, banking/finance).
Household net wealth skyrockets from about $100,000 at the median to $425k-$450k at the 25% line (and to $1.2 million at the 10% line).
Just consider software developers in the US. 1.25 million of them with a median income of $103,000 for 2017 (per the BLS). So there's probably ~650,000 software developers in the US earning six figures for 2019. That's more than the rest of the world combined.
The biggest downside to being in the US if you're in the top 25%, is that many of our largest cities aren't nearly as livable or safe as those in other developed nations. This also assumes a city to income match, where you're not making $90,000 trying to live well in San Francisco or New York (in which case you're generally going to struggle).
