Cakes with a lot of flavor in the cake mix might pair best with a simple glaze icing.
A glaze icing is a simple mixture of powdered sugar and liquid (such as water, milk, or juice) that is used to coat the top of a cake or other baked good. Glaze icing is thin and runny, and it hardens as it dries.
When a cake has a lot of flavor in the cake mix, a simple glaze icing can help to enhance the flavor without overpowering it. Glaze icing is also a good choice for cakes that have a lot of texture, such as cakes with nuts or fruit.
In contrast, royal icing is a thick, sweet icing that is used primarily for decorating cakes and cookies. Buttercream icing is a rich, creamy icing that is made from butter, powdered sugar, and flavorings such as vanilla or chocolate. Ganache is a rich, chocolatey icing that is made from chocolate and cream.
Therefore, a simple glaze icing would be the best choice to pair with a cake that has a lot of flavor in the cake mix.