How can I say this?
"I will never forget you" in Japanese?

Question

Answer

I would say "君のこと、きっと忘れない"

Kimi no koto, kitto wasurenai.

Japanese is not my native though, just an opinion :)

Answer

あなたのことを わすれません
Anata no koto o wasure masen.

...There are many way to say this in Japanese.

①あなたのことは わすれない
②きみのことを わすれない
③おまえのことを わすれない

① is used when there's no reason to be polite between the speaker and the listener.
② is used when the speaker is senior to the listener.
③ is used when the speaker is male and senior to the listener.

...Each other way has danger to be impolite or discriminative. So I recommend you the most polite and gender-free way "あなたのことを わすれません"