Applejack would be great because, she has the nurturing feel and tends to be the one where you can easily talk to. She could easily handle her child with a lot of backbone.
Fluttershy is too shy and scared, the child would be spoiled, a rebel or insanely shy.
Rarity might have a business and might be one of those mothers who is always busy, or she could settle down and raise her kid fine.
Rainbow Dash has not really matured that much, so she would have to be a little bit more mature, also she would have to show her softer side. But she would raise an aggressive wonderbolt loyal hyper child.
Pinkie probably would be a really good mother, since she's so cheerful and hyper. Her kids would be lucky They would get an amazing dessert and not turn fat.
Lastly, Twilight would be ok, but she freaks out over small things, and would have to learn how to chill.