Their little heads and cheeks that you could rub your nose and lips over all day, because it's the softest and sweetest thing in the world.
And when they get panicked when they're hungry, their little mouths wide open and their heads going all over the place - and then that instant calm when you've put them on the boob/bottle and they just settle in to you.
First smile, first clap, first giggle, first time they roll over, first wobbly, dainty little steps. When they get to the excited stage and just constantly wave their arms around in that cute jerky way.
The faces they make when they try new food!
One of my favourite things about my DD (now 15 mo) is when you ask her where her belly button is and she'll stop whatever she's doing, lift up her top and point to it. Or her impressions of animals - they make me laugh every time!
Or when you tell her no and she strops off and slaps her arms down to her sides in complete defiance, and then comes back seconds later with a smile on her face.
People say it's hard, and it is, but for every hard moment there is always a special/proud/funny moment to make up for it.
It is so true that you could never imagine having so much love for one person.
Three and half months to go until I have my second, and already DD comes up to my belly and strokes and kisses it :) Children are incredible.