Aye, I agree with Applejack being the best mother as she basically mothers Apple Bloom.
Pinkie would be the fun mom, despite the junk food, she'd be quite good I think.
Twilight would be caring and rather responsible, but her duties would get in the way
Rarity also looks after Sweetie Belle, though Sweetie Belle usually gets into trouble.
Rainbow Dash. I agree with the "plopping them in front of the TV" thing. But I reckon she would be good, if her daughter or son was like Scootaloo.
Fluttershy. Yes. Although caring and loving she'd have rings run around her. I think she would disapline, but apologise straight afterwards.