Tu es le/la seul(e) pour moi.
Here's a breakdown of the translation:
* Tu es - You are
* le/la - the (masculine or feminine depending on the gender of the person you are addressing)
* seul(e) - only (masculine or feminine depending on the gender of the person you are addressing)
* pour moi - for me
Here are some alternative, slightly more poetic or romantic translations:
* Tu es mon unique amour. (You are my unique love.)
* Tu es tout pour moi. (You are everything to me.)
* Je n'ai d'yeux que pour toi. (I have eyes only for you.)
The best translation will depend on the specific context and the tone you want to convey.