There is a fair bit of personal preference involved… and it also depends what kind of camera you have? Is it a full frame sensor (5Dmk2) or a cropped sensor (7D etc).
I’m not much a fan of ‘zoom’ lenses. So I would dsuggest perhaps getting a good L series 35mm (which on a 7D cropped sensor would be about 47mm) which would be a good around lens. But if you like things a bit wider. Perhaps a 28mm (which on a 7D would be a 37mm approx) which would be wider and perhaps better for shooting landscapes, or portraits where you want to see a bit more of the environment etc.
I have the 7D and a crappy zoom lens that came with my old 1000D. I bought a decent 50mm but find it a little soft in those spontaneous ‘baby moments’. Thanks for your thoughts, I’ve been digesting alot from http://blog.vincentlaforet.com/
Not sure if you’ve stumbled upon him?