Literal:
* Je t'aime beaucoup (I love you a lot) - This is a simple and direct way to express intense missing.
* Tu me manques terriblement (You are terribly missed) - This directly conveys the feeling of being deeply missed.
* Je t'aime plus que tout (I love you more than anything) - This emphasizes the depth of your feelings.
More Poetic/Figurative:
* Tu me manques à en mourir (I miss you to the point of dying) - This is a very strong and dramatic way to say you miss someone.
* Je n'arrive pas à vivre sans toi (I can't live without you) - This expresses a strong dependence on the other person.
* Mon cœur se brise de te manquer (My heart breaks from missing you) - This is a very romantic and heartfelt way to express your missing.
Adding Intensity:
You can add intensity to any of these phrases by adding words like:
* vraiment (really)
* terriblement (terribly)
* follement (madly)
* passionnément (passionately)
For example:
* Tu me manques vraiment à en mourir (I really miss you to the point of dying)
* Je t'aime terriblement et tu me manques (I love you terribly and I miss you)
The best choice will depend on the context and your relationship with the person you are speaking to.