As I was told this was a debatable topic, I figured I'd post here to let the debate ensue.
In my travels, and having lived overseas on several occasions, this is my opinion.
Many countries have high levels of personal freedom, the US probably has more freedom when it comes to speaking your mind, and of course the second amendment. But I would generally feel "free enough" in most Western European Nations, Canada, Australia, and New Zealand. However, like the US, they each have their own quirks.
As far as prosperity goes, that is a little harder to gauge. If you want to rise from poverty to wealth, then the US is the only place. But that means 70 hour work weeks, sitting, and sleeping on the floor with two roommates, and eating Raman three times a day, while trying to sneak in a few college courses here, and there. But, it can be done, people do it every day. There are things though that can hold you back, and limit your opportunities, and overall I think the US is not a country that really forgives past mistakes, black marks tend to stick to you, and it is extremely difficult to get past things like minor legal problems, credit issues, bankruptcy, having been fired from a job, substance abuse, and homelessness. I won't say people can't get past these things, it's just very difficult, and not a lot of people will understand.
As for freedom, I think the US probably edges out most nations, but it's a razor thin lead.
Prosperous...Only if you are willing to work yourself to exhaustion, and make a lot of sacrifices when young. And don't mess up, you may not recover from it.
The dog eat dog free enterprise system in the United States affords a lot of opportunity to make it, and make it big, but you will be stepping on other people's necks to get there. The good thing is those stepped on, if they work hard enough will have the chance to step on someone else. So, it's fair I guess
As far as the best nation goes...heck yes, but I'm red, white, and blue to my soul.
But we do have our fair share of problems, however, if you were born on a dirty landfill in a fourth world country and someone gave you a choice to immigrate to any country in Europe, most people would say "why can't I go to America"
It is what it is.