I would say '"나는 네(니)가 좋다(or 좋아) " to my close friend. If I'm going to say it to the friend I like(not just as a friend), I would say "나 너 좋아해". If the listener is older or you can't figure out which form to use, you can say '전 ㅇㅇ(name)씨가 좋아요.' in general.