Hi, warmer - if there are 2 choices warmest - if there are more than two choices Clive
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AnonymousIn this case "warmest" is the better option. However, the question could benefit from a complete rewording."Of the following materials, which is the best insulator: thermos, aluminum, or plastic?"notice the use of "best" instead of "better"I believe the original poster had in mind two different types of thermoses.