Americans are friendly in certain regions and very cold in others. But the absolute firendliest I've met in the South (Arkansas and Kentucky and rural Virginia for reference, very similar cultures between them), are still more polite than friendly. Good people, will help you out and invite you for dinner etc, but still very reserved.
Friendliest I've encountered has to be the Vietnamese. Nearly anyone on the street or in cafe or at dinner, you just wave and say xin chào and have open body language, they will have the brightest smile and invite you to hangout.