"Êtes l'amour de ma vie"
However, it's quite formal and not usually used in everyday conversation.
Here are some more natural and romantic ways to express this in French:
* "Vous êtes l'amour de ma vie" (You are the love of my life) - more formal but still romantic
* "Tu es l'amour de ma vie" (You are the love of my life) - informal, used with someone you are close to
* "Mon amour pour toi est éternel" (My love for you is eternal)
* "Je t'aime plus que tout" (I love you more than anything)
* "Tu es tout pour moi" (You are everything to me)
* "Tu es la seule personne que je veux à mes côtés" (You are the only person I want by my side)
Choose the expression that best suits the context and your relationship with the person.