Formal:
* À jamais dans nos cœurs: This is the most literal translation, but it sounds a bit formal and poetic.
* Pour toujours dans nos pensées: This translates to "Forever in our thoughts" and conveys a similar meaning.
Informal:
* On ne t'oubliera jamais: This translates to "We will never forget you."
* Tu resteras toujours dans nos cœurs: This translates to "You will always be in our hearts."
More poetic:
* Ton souvenir vivra éternellement en nous: This translates to "Your memory will live eternally in us."
The best option depends on the context and the tone you want to convey.