Flattery is defined as insincere praise. Until the 19th century, it was commonly thought that imitation was the sincerest form of flattery. Since this time, other methods of flattery have come into practice that are arguably more sincere. The problem with finding the sincerest form of flattery is that flattery by definition is insincere. So something has to have just the right amount of sincerity to be sincere, but not be sincere enough to not be insincere, and therefore not be considered flattery.