If you’re looking for a nice thing to say to a girl, I’ll make it very simple by giving you one thing to say if you’re in a relationship with her and one thing to say if you’re not.
The best example of a nice thing to say to a girl that you are in a relationship with is, “I love you,” or variations of that. There is little else that will make her happier, make her smile as wide, and make her more likely to love you back. If you are not in a relationship with the girl, one of the best things to say is, “You are beautiful,” or variations on that. This is also guaranteed to make her happy, make her smile, and be more likely to like you.
If you’re in a relationship with a girl it’s an important part of your job as her boyfriend to make her feel loved, appreciated, unique, and special. This comes from your actions more than your words, of course, but your words certainly help, and the best way to use them is to tell her you love her often. Besides simply saying “I love you,” (inflection is key to making it sound as sincere and loving as possible), it can be more effective to say things like “I love you so much,” “I love you more than I ever thought it was possible to love anyone,” “I love you so much… I only have to think about you for a second when you’re not with me and I smile to myself… I’m so lucky to have you,” etc.
The purpose should be to never make your words seem like mere words; rather, they should seem like an extension of how you really feel, and a way of expressing something towards her that was just bursting to come out of you. This will make her feel exactly how she should in a relationship with you, and you will be much more likely to make it last, and make it a great one.
If you’re not actually dating her, then a nice thing to say to a girl is, “You are beautiful.” Like saying I love you to your girlfriend, there are many better ways to tell her she is beautiful. Telling her, “You look so gorgeous when you smile,” or, “You’re eyes are so beautiful today,” or, “I can’t seem to look at any other girl when you are in the room, you just have the most beautiful smile, you just light up the room…” etc, is a more effective way of achieving what you want to say by saying any of this. Making her feel special, making her think about you in a romantic way, and making her view you as confident and genuine, because you have the balls to say such things directly to her. Saying these things to a girl will do wonders towards making her attracted to you.