What is your best chat up line?
This depends on what the situation is at the time.
Should you be in a public place, such as a bar, per se, then words are not necessary.
Should you be in a different public place, such as a Library or a grocery store, or department store and feel the need to try and pick someone up in that location?
Then it would be important to make them interact with you, such as getting their opinion on something, by asking them to judge whatever item you are thinking of getting for yourself, such as a Shirt(man/woman)...ask them how it looks on you? Their opinion opens the door to more conversation, should they participate.