I use a simple, 24 oz. Embark stainless steel bottle with a carabiner on the cap - the type you might find at a CVS Pharmacy. I clip it to my bag and sometimes bungie it down to prevent it from rattling about. Fits in my car's cup holder, or I just toss it over my shoulder into the backseat until I fill it again. I prefer my water as close to ice-cold as possible; plastic bottles, even more expensive ones with freezable inserts, just simply didn't do it for me.