Formal:
* Je t'aimerai toujours. (I will always love you.) - This is a strong and enduring declaration of love.
* Je t'aime à jamais. (I love you forever.) - This is a very romantic and poetic expression, implying an eternal love.
Informal:
* Je t'aimerai toujours et à jamais. (I will always and forever love you.) - This combines both the formal and informal options, emphasizing both duration and intensity.
* Je t'aime pour toujours. (I love you forever.) - This is a more direct and simple way to express unwavering love.
Other options:
* Mon amour pour toi est éternel. (My love for you is eternal.) - This emphasizes the eternal quality of your love.
* Tu es mon amour pour toujours. (You are my forever love.) - This emphasizes the object of your love.
The best option will depend on the context and the level of formality desired.