Formal:
* Os gwelwch yn dda: This is the most formal way to say "please." It literally translates to "if you see it good."
* Yn garedig: This translates to "kindly" and is also quite formal.
Informal:
* Rhagfabor: This is a more casual way to say "please." It translates to "in favor."
* Croeso: This literally means "welcome" but can be used informally to mean "please."
In specific requests:
* Bydd mor garedig: This translates to "be so kind" and can be used to add a polite touch to a request.
* Os gwelwch yn dda: This can also be used in specific requests and is more formal.
The best choice for you will depend on the specific situation and your relationship with the person you are talking to.