Picking a “default” is always difficult. In this particular case, you have to choose on the axis going from “fast” to “accurate”. Larger models, like full RoBERTa models, are more accurate but slower. So for demos or simply as a default value, distilbert is a good choice.
As a user you can still change which model to use 3, so you can do
pipe = pipeline("sentiment-analysis", "roberta-large-mnli")