I say Applejack.
Fluttershy likes animals, not sure if she likes fillies and stallions too. Rarity sometimes gets too annoyed with Sweetie, but... uh, guys, your Rarity theory creeps me out Pinkie... uh, well I guess accidents happen xD I think Twilight would like a more professional job, maybe she'd have to travel a lot or something :/ And she wouldn't be able to take care of a foal. Dash is just Dash. If she becomes a mother, she'd either scare her fillies/foals to death by doing a lot of stunts, or teach her fillies/foals those stunts
Applejack is more family oriented. She has a lot of family and she cares a lot about them. IMO, she would be the first one, but even if she isn't, she'd probably be the best mother.