Formal:
* Je t'aime beaucoup. (This is the most common and direct translation.)
* J'ai beaucoup d'affection pour toi. (This is a slightly more formal and less intense way to say it.)
Informal:
* Je t'aime énormément. (This emphasizes the intensity of your love.)
* Je t'adore. (This is a more affectionate and passionate way to say it.)
* J'ai un amour fou pour toi. (This is a very passionate and expressive way to say it.)
Other options:
* J'ai un profond amour pour toi. (This highlights the depth of your love.)
* Je suis très amoureux(se) de toi. (This emphasizes the strong feeling of love.)
The best translation will depend on the specific situation and the relationship between the speaker and the person they are speaking to.