Formal/Literal:
* J'aime beaucoup. (This is the most common and direct translation.)
Informal/Emphasizing the depth of love:
* Je t'aime beaucoup. (This is the same as above, but using the informal pronoun "tu" for a closer relationship.)
* Je t'aime très fort. (This emphasizes the strength of your love.)
* J'ai beaucoup d'amour pour toi. (This emphasizes the quantity of your love.)
More poetic/literary:
* Je t'aime à la folie. (This means "I love you madly.")
* Mon amour pour toi est immense. (This means "My love for you is immense.")
The best translation depends on the context and the relationship between the speakers.