The quest for the best buttercream is as subjective as saying which is my favourite colour.
But I am going to do it anyway :-) . Because there are some varieties and all can be used in reality but a couple of them are in my opinion better. It's the case of Italian merengue buttercream and swiss buttercream. There are two big differences to the traditional buttercream. First there is no icing sugar involved and that creates the two differences, the level of sweetness and the texture. Swiss and italian buttercream are by far smoother and less sweet than traditional buttercream (including the cream cheese buttercream).
Personally I feel that unless the cake is not particularly sweet, I wouldn't use all butter buttercream. Simply because a cake with icing needs to be balanced and that can only happen if one complements the other. If the cake is rich and sweet and the icing is also sweet then it's an overkill of sugar. Some people obviously with a sweet tooth may find this absolute heaven, but I think it's not a well balanced cake.
Additionally, the texture of IMB and SMB is amazingly smooth, like velvet and the somewhat gritty texture of icing sugar doesn't exist which creates that luscious feel in our mouth.
But testing recipes is to say the least, challenging. I have never been a big fan of butter. I can eat a toast (gluten free) with butter but it needs to be unsalted. I absolutely hate salted butter. But, I must add, my family is weird. Both my mother and sister don't touch any dairy beyond a particular cheese in years. I am more flexible, and tasting the several stages of buttercream is done in the name of baking art. My favourite part is the end stage adding the flavour and completely changing the appearance, texture and flavour. That's when the buttercream comes alive. Today I made chocolate italian merengue buttercream and it's divine. I am going to use it in some chocolate gluten free cupcakes. Watch out for the photos...I can guarantee you cannot tell the difference between a cupcake made with normal wheat flour and these ones. They are rich, moist, soft and full of flavour. In contrast the buttercream is smooth, with a hint of chocolate but not overly sweet. Simplicity and quality.
I better hit the treadmill tomorrow....the taste of chocolate lingers!