I think I'd have to say Asians overall. Smaller average size = less demand for resources = more efficient. I can't remember where I saw it, but I seem to also remember that Asians are the newest racial group, followed by whites, and finally blacks as the oldest.
I mean nothing racist in this regard either, it's just a matter of objective reasoning. Life has a way of adapting to resources and demands. For example healthy individuals of african descent have on average very little subcutaneous fat, which is ideal for high temperature and solar exposure, makes cooling very efficient, but it's terrible for cold temperatures. Average healthy individuals of white/european descent will have a great deal more subcutaneous fat, which provides valuable insulation for surviving cold weather, but is a problem with extreme heat.
It's all very fascinating stuff, and we will learn a lot more as genetic analysis improves. I just hope that racism and unscientific propaganda doesn't trump the larger lessons we can learn. The more we know about the differences between racial groups, the better medical care is possible in targeting problems that strike certain people much more often than others.