[duo797]

愛してる
or
(name)が大好き

Personally I'd go with the second one, if you give me a name I can do that part for you too (in katakana). The first one, as I've read, is too direct of an 'I love you' and something you don't hear very often. The second is 'I like you a lot' but you can pretty much construe that to be love (not as nitpicky as some english people can be, I like you a lot in a romantic way but I don't love you. Don't need to worry about that kinda distinction). In case you're wondering, they're pronounced with romaji:
ai shiteru
(name) ga daisuki