Which ingredient is NOT part of a Perfect Manhattan?

In a Perfect Manhattan, the traditional recipe calls for the use of both sweet and dry vermouth along with a base spirit, which is commonly bourbon or rye whiskey. Specifically, the Perfect Manhattan uses a dash of both dry vermouth and sweet vermouth to achieve its distinct flavor balance.

While bourbon is a traditional ingredient in a Manhattan, tequila does not belong in this cocktail. The combination of the two types of vermouth provides the 'perfect' twist to the classic drink, making it more complex than just using sweet vermouth alone. Therefore, the inclusion of tequila deviates from the classic preparation and flavor profile of a Perfect Manhattan, marking it as the ingredient that is not part of this cocktail.