Here are a few ways to express it:
Formal:
* Спасибо моему ангелу-хранителю (Spasibo moemu angelu-khranitelyu) - "Thank you to my guardian angel"
Informal:
* Спасибо тебе, мой ангел! (Spasibo tebye, moy angel!) - "Thank you, my angel!"
* Ты мой ангел! Спасибо! (Ty moy angel! Spasibo!) - "You are my angel! Thank you!"
More specific:
* Спасибо за твою помощь, мой ангел. (Spasibo za tvoyoo pomoshch, moy angel.) - "Thank you for your help, my angel."
The best way to express this sentiment depends on the context and your relationship with the person you are thanking.