Which ingredient tops off a Harvey Wallbanger?

The ingredient that tops off a Harvey Wallbanger is Galliano. This cocktail is traditionally made by mixing vodka and orange juice, then finishing it with a float of Galliano, which is an Italian herbal liqueur characterized by its sweet and aromatic profile. The use of Galliano not only adds a distinct flavor but also a beautiful golden color that enhances the visual appeal of the drink.

In contrast, the other ingredients listed do not play a role in the classic recipe for a Harvey Wallbanger. Grenadine is a sweet syrup that would change the nature of this particular cocktail, peach schnapps is typically associated with different fruity cocktails, and Southern Comfort is a fruit-flavored whiskey that is not part of the Harvey Wallbanger's traditional ingredients.