Formal:
* Obrigado, meu amor. (Male speaker)
* Obrigada, meu amor. (Female speaker)
Informal:
* Obrigado, meu bem. (Male speaker)
* Obrigada, meu bem. (Female speaker)
* Valeu, meu amor. (Male speaker, very informal)
* Valeu, meu bem. (Female speaker, very informal)
More affectionate:
* Obrigado, meu amor da minha vida. (Male speaker)
* Obrigada, meu amor da minha vida. (Female speaker)
* Obrigado, meu anjo. (Male speaker)
* Obrigada, meu anjo. (Female speaker)
You can also use these phrases to express your gratitude and affection:
* Te amo! (I love you!)
* Você é tudo pra mim. (You are everything to me.)
* Obrigado por tudo. (Thank you for everything.)
Choosing the right expression depends on your relationship with the person and the context of the situation.