Here are some options, depending on the nuance you want to convey:
* Ffrind gorau: This is the most literal translation, meaning "best friend."
* Ffrind agosaf: This translates to "closest friend" and implies a deeper level of connection.
* Ffrind cysgodol: This means "shadow friend" and implies a very close and inseparable relationship.
* Ffrind enaid: This translates to "soul friend" and emphasizes a deep emotional connection.
Ultimately, the best way to express "best friend" in Welsh will depend on the specific relationship you want to describe.