Je t'aime beaucoup.
Here's a breakdown of the phrase:
* Je: I
* t'aime: love you (informal, used with someone you are close to)
* beaucoup: very much
Other options:
* Je t'aime à la folie: I love you madly (more intense)
* Je t'aime plus que tout: I love you more than anything (more emotional)
* Je t'aime de tout mon cœur: I love you with all my heart (very romantic)
The best choice will depend on the specific context and the relationship between the speakers.