re: example
If I'm reading you right, I haven't seen very many major nations whose politics are sterling examples of human behavior, so I don't think the U.S. is anything special. There's something about the very process of democracy that tends to select for those who appeal to the least common denominator.
Wish I had a better system, but I don't. Unless you count my "draft the politicians" idea.
I think you have to normally get down to the level of Duchess of Grand Fenwick before you start getting to consistently decent leaders, although some small nations have leaders that are also much worse.
I've been favorably impressed by Prince Harry, lately.