Formal:
* Vous me signifiez beaucoup. (You mean a lot to me.) This is a very formal and literal translation.
* Vous êtes très important(e) pour moi. (You are very important to me.) This is a more common and natural way to express this sentiment in a formal setting.
Informal:
* Tu comptes beaucoup pour moi. (You mean a lot to me.) This is a more casual and personal way to say it.
* Tu es très précieux(se) pour moi. (You are very precious to me.) This is a more affectionate and intimate way to say it.
* J'apprécie beaucoup ta présence dans ma vie. (I really appreciate your presence in my life.) This emphasizes the importance of their presence in your life.
More emotional:
* Tu es tout pour moi. (You are everything to me.) This is a very strong and passionate expression of love.
* Je t'aime beaucoup. (I love you a lot.) This is a more direct and affectionate way to express your feelings.
The best way to say it depends on the context and your relationship with the person you are talking to.