Formal:
* Je vous manquerai. (pronounced: "zhuh voo man-kray") - This is the most formal way to say it and should be used with someone you don't know well or who is older than you.
Informal:
* Je vais te manquer. (pronounced: "zhuh vay tuh man-kay") - This is the most common way to say it and is used with friends and family.
* Tu me manqueras. (pronounced: "too muh man-kra") - This is also informal and means "You will be missed by me."
More emotional:
* Je t'aimerai toujours. (pronounced: "zhuh tem-ray too-zhoor") - This means "I will always love you" and is a very emotional way to say goodbye.
The best way to choose the right phrase depends on your relationship with the person you are talking to.