Formal:
* Tu me manques beaucoup. (You are missing to me a lot.) - This is a simple and direct way to say it.
* J'ai tellement envie de te revoir. (I have so much desire to see you again.) - This emphasizes your longing to see the person.
Informal:
* Tu me manques trop! (You miss me too much!) - This is more affectionate and playful.
* J'ai tellement envie de toi! (I have so much desire for you!) - This is very affectionate and can be used with a romantic partner.
More poetic:
* Ton absence me pèse. (Your absence weighs on me.)
* Je pense à toi constamment. (I think of you constantly.)
The best way to say it will depend on your relationship with the person you are speaking to.