An absolutely insane amount. It's ridiculous just how wealthy and the quality of life the average American has compared to the world.
> Are Americans, on average happier and have better life outcomes than other developed nations?
Yeah for the most part they are in the same ballpark.
I've been there last year. This is absolutely not true compared to Europe, including post-soviet states. Might have been true a few decades back maybe. Of course, we can argue that the US citizens have it made compared to someone in Kenya (do they?) but that's not the spirit of the question, is it?
Is there another country that comes close?
Sure, if you're from say, Haiti, even the US will look very attractive, but the bar is pretty low there, wouldn't you say?
(e.g. backing and installing dictatorships[1], contributing massively to climate change, ...)
[1] https://en.wikipedia.org/wiki/Military_dictatorship_of_Chile
In fairness the same has happened in the US recently.