Formal:
* Je ne vous oublierai jamais. (This is the most literal translation and is appropriate for formal situations.)
* Votre souvenir restera à jamais gravé dans mon cœur. (This means "Your memory will forever be engraved in my heart." It's a more poetic and emotional way to say it.)
Informal:
* Je ne t'oublierai jamais. (This is the informal version of "Je ne vous oublierai jamais.")
* Tu resteras toujours dans mon cœur. (This means "You will always be in my heart." It's a very romantic and heartfelt way to say it.)
* On ne se oublie pas comme ça. (This means "We don't forget each other like that." It's a more casual and friendly way to say it.)
Other options:
* Je me souviendrai toujours de toi. (This means "I will always remember you.")
* Tu as une place spéciale dans mon cœur. (This means "You have a special place in my heart.")
The best way to say it depends on the specific situation and your relationship with the person you are speaking to.