What would you show of humanity to aliens if you could only show one thing?
We're talking the fact that our entire race's survival rests on this show and tell, right?
First, and most important, I'd learn a bit about the alien society: what do they value, what do the aliens hold up as an ideal?
Then whatever that ideal is, I'd show them a human baby and it's ability to become any ideal over time. We're super malleable. That's a huge value. No matter how difficult we humans might be at the moment, just wait around for a few decades visibly watching us but not interacting or saying anything beyond "hmmmmm..", and we'll become super obedient.