Formal:
* Je vous manque. (pronounced: zhuh voo mahnk) - This is the most formal way to say "I miss you" and is used for someone you respect or don't know well.
Informal:
* Tu me manques. (pronounced: too muh mahnk) - This is the most common and informal way to say "I miss you" to a friend or someone you know well.
* Je t'aime beaucoup et je te manque. (pronounced: zhuh tem boo-koo eh zhuh tuh mahnk) - This literally translates to "I love you a lot and I miss you" and is a more affectionate way to say "I miss you."
You can also use "Je pense à toi" (pronounced: zhuh pahns ah twah) which means "I think about you." This conveys a sense of longing and missing someone.
The best way to choose the right phrase depends on your relationship with the person you are talking to and the context of the conversation.