Formal:
* Ako ay sa iyo. (Formal and literal: I am to you.)
* Sa iyo ako. (Formal and more poetic: To you I am.)
Informal:
* Ikaw lang ang akin. (You are the only one for me.)
* Sayo lang ako. (I am yours alone.)
* Ikaw ang mahal ko. (You are the one I love.)
Romantic:
* Ikaw ang lahat sa akin. (You are everything to me.)
* Para sa iyo ako. (I am for you.)
The best choice will depend on your relationship with the person and the tone you want to convey.
For example, if you are saying it to your significant other, you could use the romantic options. If you are saying it to a friend, you could use the informal options.