I believe 50/50 is optimal overall, considering boiling point, corrosion protection, low surface tension, pH, etc. However, from a pure heat transfer point of view, pure water has higher heat transfer coefficients than a mixture. That's why race cars use straight water and some people run 60(water)/40(antifreeze) if they don't need good freeze protection. If you keep adding antifreeze and you get too far above 50/50 it will start to reduce cooling.