I believe the heat transfer capacity of water is much better than antifreeze. For maximum heat transfer, you'd run 100% water. However, you need some antifreeze to keep the water from freezing (naturally) and also it serves as a water pump lubricant (to some small extent) and it has corrosion inhibitors.
By running 100% antifreeze, you impair the heat transfer capabilities of the coolant - you might have "hot spots" near the cylinder walls that are not reflected in the temperature gauge.
In most cases 50%/50% mix of water/antifreeze works well.