For as long as PC enthusiasts have been building water loops the question of "what liquid they should use" has been at the forefront. Asetek said it best, Distilled Water. That is by far the best fluid to use at least when it comes to heat capacity.

As we know things like algae growth and corrosion can be a problem when using pure water and is why we now have specialized fluids for our PC water loops. You also shouldn’t be surprised that the primary ingredient of these coolants is distilled water due to its ability to store massive amounts of heat without a sharp rise in temperature. It all has something to do with "specific heat" so I'll let you research that on your own.