Anata no koto ga suki da. = I l ike you (romantically).
The expression here requires "koto".
To tell you the truth, I don't know why. But if you said: "anata ga suki", it doesn't mean "romantically".
If you like your friends at school you say: "suki".
If you like the hot girl next to you you say: "~no koto ga suki."