Formal:
* Ffrindiau da: This is the most literal translation and means "good friends." You could use this when talking about someone you respect and admire.
* Ffrindiau agos: This means "close friends" and implies a strong bond.
* Ffrindiau dibynadwy: This means "reliable friends" and emphasizes trustworthiness.
Informal:
* Ffrindiau gorau: This translates to "best friends" and is used for very close friends.
* Ffrindiau annwyl: This means "dear friends" and conveys affection.
* Ffrindiau cyd-ddigonol: This literally means "friends who are enough together" and is a more playful way of expressing close friendship.
Ultimately, the best phrase to use will depend on the specific situation and the tone you want to convey.