Pretend time.
There is a meeting of all sentient life forms in the universe that are able to communicate. In it they try to decide what is the best thing each dominate species of their respective planets have produced. The focus comes to humans. So what do you think aliens would say is humanity's greatest gift to the universe?
We have technology to consider, language etc etc. But when it comes down to it, something that is unique and would make us stand out for the "others", I think it would be hard to go past music. Music is our greatest creation.
Thoughts?