I'll give you a hint: it's not "Send me a picture of your boobs." Dating site Badoo tracked the success rate of opening lines in users' messages (in eleven different languages, even!), and it found that the most successful chat-up is "You have beautiful lips." Which, frankly, sounds a bit serial-killer to me. Anytime a stranger is focusing a lot of attention on a specific body part, my first thought is, "It puts the lotion in the basket or it gets the hose again." Clearly much of the world disagrees.
However, different lines took the top spot in different countries. Here in America, women — and it seemed this study only looked at men messaging women — liked "You dress beautifully." (And I agree with my countrywomen.) Different places liked compliments on their legs, hair, etc. But the weirdest was probably "You have beautiful ears," which you should use if you're courting a Dutch woman.
Sorry girls, this study has nothing for you! But at least you'll always have this: