Formal:
* Je vous manque. (This is the most formal way to say it, used for someone you don't know well or someone older than you.)
Informal:
* Tu me manques. (This is the most common way to say it to someone you know well.)
* Je t'aime beaucoup et je te manque. (This is a more affectionate way to say it, meaning "I love you a lot and I miss you.")
* J'ai hâte de te revoir. (This means "I can't wait to see you again" and is another way to express missing someone.)
Other options:
* Je pense à toi. (This means "I'm thinking of you." While it doesn't directly translate to "I miss you", it conveys a similar sentiment.)
* Il me manque ton sourire. (This means "I miss your smile." This is a more specific way of saying you miss someone, highlighting a particular aspect of them.)
The best choice will depend on your relationship with the person you're talking to and the tone you want to convey.