Here are a few options, ranging from playful to more direct:
Playful:
* "I'm not sure what's more beautiful, the sunset or your smile."
* "You seem to know how to make a good impression. I'm impressed."
* "I was just thinking about how much I like your [hair/eyes/sense of humor]. Is that okay?"
Direct:
* "I've been wanting to talk to you all night."
* "You're even more interesting in person."
* "I think you're really attractive. Would you like to get a drink sometime?"
Remember:
* Be confident but not cocky.
* Read the other person's body language. Are they receptive to your flirting?
* Keep it light and fun.
* Be respectful.
Ultimately, the best flirt sentence is one that feels natural to you and is appropriate for the situation. Good luck!