Formal:
* Elle me manque. (This is the most common and straightforward way to say it.)
Informal:
* Je l'aime beaucoup. (This translates to "I love her a lot," but it's often used to express missing someone.)
* Elle me manque vraiment. (This is a more emphatic way to say "I miss her really badly.")
* Je pense beaucoup à elle. (This translates to "I think about her a lot.")
The best way to say it will depend on your relationship with the person you're speaking to and the context of the conversation.