Arguably one learns it by looking at facts (and yes, early indoctrination). Third largest country by population in the world, vast natural resources, incredible human diversity, largest GDP in the world (#2 is China which is lower by 30%), largest immigrant population (next in line is Germany, which is lower by 60%), and so on. Sure, there is always room for improvement. The founders wrote in principles of human equality, despite being slaveowners, I imagine in the same way a smoker thinks from time to time about quitting. And within a century slavery was indeed abolished here. Point being, I think highly subjective terms like ‘best’ allow one to consider potential for greatness. Much about the USA, on personal as well as national levels, is that there is room for ambitious aspirations to actually materialize.
Now, that said, imagine if the USA were the best car you could think of–powerful, safe, reliable, whatever–but it is driven by selfish drunks prone to road rage. On paper, and often engraved in granite walls, there appears little room for improvement. But humans being humans, the machinery is not allowed to function as designed. And so it becomes sort of a hoopty.