This is easy: Americans over 35 or so are the best volunteers overall based on my experience here. We would be nowhere without them. Younger Americans are pretty good too but they are kinda a different breed compared to over 35 ones and would not take the first spot on their own. All Americans mixed together would still take the first spot though I think (because Americans over 35 were really damn good).
Yeah, I know that "everyone is different", "splitting people by nationality is bad", etc, etc but I don't care about that stuff. And I like jokes based on stereotypes too.