Well no, it is not. Productivity cannot be measured in GDP due to various (political, military, ownership/control of resources etc.) factors skewing international trade that it's derived from.
Anyway, Americans are obviously not top country vis-a-vis GDP. And that is after the fact that USA is the big-dog of international relations that doesn't shy from imposing control over someone else's resources using military aggression (Afghanistan, Iraq, Libya to name a few), which is hardly true for places like Luxembourg or Norway.