Not to get too dripy and mushy, but the best sound in all the world has to be a baby laughing.
To me, it's Little M's laughter. It sounds like he's saying, "Hee, hee, hee", but with a sort of bell-like quality and just a little tinge of dorkiness thrown in there. It's just magical.
I don't remember the date when I first heard him laugh, but I remember what circumstances brought it about. I had Little M on the changing table in his room getting ready to go take a bath and I needed to distract him from trying to turn over so I could get his clothes off.
I used "Here comes Ookie, giddy, giddy, giddy!" (Grampy and other family should appreciate that!) and tickled his tummy and he just belted out that laughter! And it got him all wound up, which was probably not the wisest move before putting him in water, but the bathroom will survive.
Now I use that tickling and laughter to keep him entertained every day, and I probably spend about half my time with him trying to make him laugh. He laughs at funny faces, surprised faces, peek-a-boo, the cats, and lots of other things we haven't even discovered yet. He's even been trying to blow raspberries on my arm like I do on his tummy. He's one of the happiest kids I've ever seen.
And that makes me one of the luckiest Moms ever, and I thank God every day for entrusting me (and Big M) with raising this wonderful child.