Formal:
* Vous me rendez heureux/heureuse. (You make me happy.) - This is the most formal way to say it.
* Vous me procurez du bonheur. (You bring me happiness.) - This is also a formal way to say it.
Informal:
* Tu me rends heureux/heureuse. (You make me happy.) - This is the informal version of the first option.
* Tu me fais plaisir. (You give me pleasure.) - This is a more common way to say it in informal settings.
* Tu me rends joyeux/joyeuse. (You make me joyful.) - This is a slightly more emotional way to say it.
* Tu me rends heureux/heureuse comme un enfant. (You make me happy as a child.) - This is a very endearing way to say it.
You can also use an adjective to describe the kind of happiness you feel:
* Tu me rends très heureux/heureuse. (You make me very happy.)
* Tu me rends tellement heureux/heureuse. (You make me so happy.)
* Tu me rends incroyablement heureux/heureuse. (You make me incredibly happy.)
The best way to say it will depend on the context and your relationship with the person you are speaking to.