私にとってのよい友達 を英語に訳すと？

どちらも正しいですが、意味が少し異なります。

a good friend to me
(good to me)
私に優しい、仲良くしてくれる友人

a good friend for me
(good for me)
私のためになる友人。付き合っていると自分に利益がある友人。すごく仲が良いとは限りません。親が子に He is a good friend for you. と言うこともあるでしょう (彼が優秀なので、付き合っているとお前に良い影響がある)。

The doctor is good (nice) to me.
This medicine is good for me.

I like Angela. We practice tennis together. When I lose a match, she always talks to me and walks home with me. She is a good friend to me.

I like Chris. He is good at tennis. I want to be able to play tennis like him. He motivates me to practice every day. He is a good friend for me.