Formal/Neutral:
* Partner
* Significant other
* Sweetheart
* Beloved
* My love
* My lady
* My better half
* My girl (can be informal)
Informal/Affectionate:
* Babe
* Honey
* Love
* Darling
* Sweetie
* Boo
* My girl
* My woman
* My everything
* My best friend
More playful/lighthearted:
* My main squeeze
* My ride or die
* My soulmate
* My partner in crime
* The one
* My other half
Specific to the relationship:
* Fiancée (if engaged)
* Wife (if married)
Remember, the best choice depends on your relationship with the person and the context of the conversation.