* Ευχαριστώ, αδερφέ (Efharistó, aderfé): This is a literal translation. "Efharistó" means "thank you," and "aderfé" means "brother." This is perfectly acceptable and widely understood.
* Σε ευχαριστώ, αδερφέ μου (Se efharistó, aderfé mou): This adds "μου" (mou), meaning "my," making it "Thank you, my brother." This is more affectionate.
* Χίλια ευχαριστώ, αδερφέ (Chiliá efharistó, aderfé): This translates to "a thousand thanks, brother," expressing deeper gratitude.
* Using a more informal term of endearment instead of "αδερφέ" (aderfé) might also be appropriate depending on your relationship, for example, a nickname.
The best choice depends on the situation. For most everyday situations, Ευχαριστώ, αδερφέ (Efharistó, aderfé) is perfectly fine.