Formal:
* Hwylus iawn: This is a literal translation of "very nice," and is appropriate in formal situations.
* Yn rhyfeddol dda: This means "wonderfully good," and can be used when you want to express a high level of appreciation.
* Yn arbennig o dda: This means "especially good," and is also appropriate in formal situations.
Informal:
* Braf iawn: This is a more casual way to say "very nice," and is suitable for everyday conversations.
* Cwtch: This is a word that encompasses the feeling of being cozy and comfortable, and can be used to express the sentiment of "very nice" in a warm and friendly way.
* Gwych: This means "fantastic," and is a more enthusiastic way to say "very nice."
The best way to choose the right phrase depends on the specific situation. If you are unsure, it is always best to err on the side of formality.