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.
A guy who is not her boyfriend can say anything sweet to a girl that is not sexual. Example: "You're beautiful," "You're Intelligent," "You're Fascinating" whatever stay in that area. Make sure it is true though.
it has to come from the heart. at the right moment. check out the short movie "signs"
there is no certain phrase...its something specific about the girl.
like, this guy friend of mine told me that I'm the best artist he knows & that he hopes I could draw/paint something for him someday. it was really sweet. a simple compliment or a bit of honesty about how you feel can make a girl's day :)