I think the key is sincerity and scarcity, a more a man compliments people the more I distrust them, or believe them to be easily handed out, if i've known someone for a year and they say 'you're really smart' it's going to have more impact than if i've known you for 5 minutes. As for appearance, a girl likes to be desired, if she looks good say so, but don't overdo it. I'd say girls like more specific compliments that say something unique about them. Sexual compliments are fine as well, sometimes a girl doesn't want to be told she's modest or nice, sometimes she wants to know a man wants her.