i'm not a girl, but i can give you advice. it depends on the types of girls. for example: control freaks like to be in control, so hearing something unexpected or something she can't control could make her mad.
girls who hear all the time that they're hot, don't like hearing it over and over again. girl's who are not, could have their greatest day if someone says she's beautiful.

there are some things which girls generaly like to hear. most of these things create a "aah that's so cute/ sweet!" reaction, while laughing hard and/or smiling.

to the girl who hears (and knows) she's hot all the time, you beter says something special. Most hot girls are laughed about that they're superficial, so saying something like "you're genuine, because ...", "you're quite intelligent, because ...". you should listen carefully to what she's says, because each time you complement her on something or you qualify her, you HAVE TO give her a reason WHY she is genuine, intelligent, filosofic, ... etc.

you can give it a funny twist, by implenting a neg or cocky funny in it. you can say: "ALTOUGH you ..., you are ... because ... and i like you for that". first she might laugh hard, because you teased her a bit, then she will listen carefully to what to what you say, still smiling, and you qualify her, and she would awnser "ooh that's so sweeeet".

for example: "altough you can't help sitting on my cock all the time and you spit on me, (small pause) i think you're intelligent, because of you're general knowledge and your funny view on the world, and i like you for that." (and if you want you can add): "you're adorable! i'm sooo gonna adopt you as my new little sister!"

note that you should only compliment a girl if she deserves it.

Q: What girls like to hear about
A: Themselves!

Cute, pretty, sexy, hot, fine - all useless and contrived. In one ear and out the other.

In an incredibly unscientific poll I made up for this response, 100% of girls love being told they are "beautiful". The B-word is the magic key. "Gorgeous" is a close second.

Best uses include:

1) morning after sex. "Good morning, beautiful"
2) when she feels she's being played. "every guy at this bar is jealous because I'm talking to such a beautiful creature"
3) as part of a neg. "beautiful girls always talk too much."
4) when she is trying to be slutty-hot, therefore trying to figure out whether your interest is pure or purely sexual. "you look beautiful"