For me it's when Ah Jai (my 3-month baby boy) jumps forward and crouches down with his butt in the air, smiling all the while. :-D He does it whenever he wants to play. Such a shame I've never been able to capture it with my camera.
Oh, and the lovely head tilt that he does whenever he's confused as to what I just said.
And the way he licks his smiling lips when he sees his food bowl.
I can just go on and on...
Dino does not talk back but to look into his eyes when it comes to dinner time...he just is not too vocal.