Distilled. It removes all the minerals in the water which could cause scale or rust.
Here in Atlanta we have water that is on the soft side without a lot of mineral content so tap water is not too bad.
Distilled water costs about a dollar a gallon though, so I always buy it at the grocery store and mix it with the ethylene glycol concentrate at a 50/50 rate.
On old cars from the <20s - 30s, you're better off with pure water in the summer and water/alcohol mix in the winter. The cooling systems aren't pressurized and some don't have water pumps. The modern antifreeze is too viscous to flow properly so alcohol, is preferred as an antifreeze additive.
