Formal:
* "Eu amo você mais que a vida." (This is the most direct translation and is quite formal)
Informal:
* "Eu te amo mais que a vida." (This is a more informal and common way to say it)
* "Eu amo você mais do que tudo." (This translates to "I love you more than anything", which is a similar sentiment)
* "Você é a minha vida." (This literally means "You are my life" and is a very strong expression of love)
The best way to say it depends on the context and your relationship with the person.