Ranking Butter

Few words have such warm and appealing connotations as butter. Just a mention of the word calls to mind comfort, sumptuousness, and fulfillment. Perhaps this is why so many smooth, pureed products choose to call themselves butter instead of the more technically accurate paste.
But what is butter? You can find butters in the refrigerated section of the grocery store, on the condiment shelf, and in the beauty aisle. But what exactly makes a butter a butter, and which butter is the best one?