For me it’s the Downy Yellow Buttercake - it tastes wonderful on its own.
And, at the moment (because of my passionfruit frenzy of baking last week) it’s the passionfruit version of the Mousseline Buttercream. But I think any of the other fruit flavours would also be wonderful. I made the chocolate version of the neoclassic buttercream last year and thought it was marvellous, but really, the Mousseline beats it hands down - when made properly, it doesn’t even taste very buttery, is so-o-o light and fluffy, that I can’t believe it’s buttercream. I need to do an orange version soon.