A question like this isn't really a matter of opinion. If cooling is your primary goal, straight water will cool the best. The problem is that straight water will corrode your water passages. Adding WaterWetter or anti-freeze will help prevent corrosion.
If you are already running an anti-freeze/water mix, WaterWetter will help with heat transfer and improve cooling.
NEVER run straight anti-freeze. Anti-freeze is not a coolant. It's what it says it is.
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