Formal:
* Gracias por ser mi amigo/a. (Thank you for being my friend) - This is the most literal translation and is appropriate for a more formal setting or when speaking to someone you don't know very well.
* Le agradezco por ser mi amigo/a. (I thank you for being my friend) - This is a more formal and slightly more elaborate way of expressing gratitude.
Informal:
* Gracias por ser mi amigo/a. (Thank you for being my friend) - This is also appropriate for informal situations, especially when speaking to someone you know well.
* ¡Gracias por ser mi amigo/a! (Thank you for being my friend!) - This is a more enthusiastic way of expressing gratitude, often used with friends or family.
* Te agradezco por ser mi amigo/a. (I thank you for being my friend) - This is a more heartfelt and personal way of expressing gratitude, often used with close friends.
Even more affectionate:
* Gracias por tu amistad. (Thank you for your friendship) - This is a more general way of expressing gratitude for someone's friendship.
* Te quiero mucho por ser mi amigo/a. (I love you very much for being my friend) - This is a very affectionate way of expressing gratitude, often used with close friends or family.
Remember to choose the appropriate expression based on your relationship with the person you are speaking to.