Formal:
* Merci beaucoup, mon amour. (Thank you very much, my love.)
* Je te remercie beaucoup. (I thank you very much.)
Informal:
* Merci, mon chéri/ma chérie. (Thank you, my darling/my dear.)
* Merci beaucoup, je t'aime. (Thank you very much, I love you.)
* Merci d'être toi. (Thank you for being you.)
* Tu es incroyable! Merci. (You are incredible! Thank you.)
Romantic:
* Je te suis tellement reconnaissant(e). (I am so grateful to you.)
* Tu es mon rêve devenu réalité. (You are my dream come true.)
* Merci d'être dans ma vie. (Thank you for being in my life.)
Remember to choose the phrase that best fits your relationship and the situation!