That depends on where you’re taking it and how much food you need to keep cool. But, generally, the best lunch box will have plenty of insulation and a secure closure to keep the cold air from dissipating, such as a zipper or a jar-like seal. More “open”-style bags with simple velcro closures are not going to have the cooling power of more secure styles. If you're taking food and drinks for a group, you may want to think about using an ice chest.