Informal:
* Je t'aime beaucoup. (I love you very much) - This is a common way to express missing someone in a romantic context.
* Tu me manques. (You are missing to me) - This is a direct translation of "I miss you," but it can sound a bit formal in informal settings.
* J'ai envie de toi. (I want you) - This expression is more passionate and implies longing.
Formal:
* Je vous manque. (I am missing to you) - This is the formal way to say "I miss you" to someone you are addressing with respect.
* Votre absence me pèse. (Your absence weighs heavily on me) - This is a more poetic and literary way to express missing someone.
For a friend:
* Je pense à toi. (I think about you) - This is a more casual way to express that you miss a friend.
It's important to choose the right expression based on your relationship with the person you are talking to.