Formal:
* Мой самый дорогой/дорогая (moy samyy dorogoy/dorogaya) - This is the most literal translation and can be used for someone you hold in high regard, like a respected elder or a close friend.
Informal:
* Мой любимый/любимая (moy lyubimyy/lyubimaya) - This is a more affectionate way to say "my dearest" and is usually used for a romantic partner or a family member.
* Моя дорогая (moya dorogaya) - This is a common and endearing term for someone you love, often used by men to address their wives or girlfriends.
* Мой милый/моя милая (moy milyy/moya milyaya) - This is a sweet and gentle way to say "my dearest" and is often used for someone you adore.
Other options:
* Мой единственный/моя единственная (moy edinstvennyy/moya edinstvennaya) - This means "my only one" and expresses a deep, unique connection with the person.
* Моя душа (moya dusha) - This means "my soul" and conveys an intense emotional connection.
The best choice for you will depend on who you are talking to and the level of intimacy you wish to express.