Easy answer from me at least, what makes me smile is seeing them happy!
I always get happy when I see they are enjoying a new toy, gulping down some new food that they tried, playing with us or when I see them in their "exploration and play"-mode. Seeing them work for treats and strengthening our bond through training is also awesome.
Leroy's progress, and the fact that he now actively seeks our companionship and that he both calls for us, and flies to us to hang out is awesome. It's so heartwarming to see how he's progressed since we got him.
That's whats making all the hardships with parrot ownerships worth it.
I know that's probably not what you meant, but these are the things that make me the most happy. To me, a specific behaviour isnt as important

.