Which coolant and mixture is best?

I run Evans waterless coolant with very good results; see here, here, and here for discussion and here for the company's website. It is costly but beneficial -- not only in terms of engine-cooling performance, but when there's no water in the cooling system there's no corrosion, and when there's no pressure in the cooling system all the hoses, solder joints, seals, and gaskets are under a great deal less stress.