Formal:
* Vous êtes mon prince. (This is the most literal translation and is very formal.)
Informal:
* Tu es mon prince. (This is the informal version, suitable for someone you know well.)
* Mon prince. (This is a more affectionate and intimate way to say it, implying a close relationship.)
* Tu es mon roi. (This literally translates to "You are my king" and can be used as a playful or romantic way to say "You are my Prince.")
Other options:
* Tu es l'homme de ma vie. (This means "You are the man of my life.")
* Tu es mon rêve. (This means "You are my dream.")
The best way to say it depends on the context and your relationship with the person you are speaking to.