GIRLS DO NOT ANSWER THIS QUESTION.
I'm not telling you because you're just going to go say it to some girl and it's not going to be sincere coming from you. You won't be saying it because you genuinely mean it, you'll be saying it because you read on the internet from some girl that it was the sweetest thing a man can say to a girl. That, in my opinion, is f***ed up. First, you don't have the originality and creativity to come up with a sincere, genuine compliment. Tnen you take credit for someone else's sincere cuteness. Then you give this poor girl the impression of something that is false just to impress her! Even if it's not false, it's still wrong because it's not coming from the heart.