The one with a rubberized lining.
The simplest way to keep your water cold is to buy a bottle that has an insulated, rubberized lining. The lining insulates heat from getting into the bottle and turning the inside into a hot greenhouse. Bottles without insulation or that do not have a rubberized lining will cause water inside to warm up–quickly–so drink it before it goes flat too! Check out ZiiP Eco-Friendly Stainless Steel Water Bottle which has both! 🙂