Kocham cię (pronounced "koh-hahm tsee-en")
This is the most common and formal way to say it.
Here are some other ways to say it:
* Kocham Ciebie (more formal)
* Kocham Cię (informal)
* Bardzo Cię kocham (I love you very much)
* Uwielbiam Cię (I adore you)
* Jestem w Tobie zakochany/zakochana (I am in love with you) - used for romantic love
The choice of which phrase to use depends on the context and your relationship with the person you're speaking to.