Hmm, decisions...Rainbow Dash has a special mention, just because she's so athletic and puts everything into all she does. I suspect she'd be a delicious little fighter, fighting back every minute and always eager for more.
But since they're ponies, I'd have to go with Rarity. Sensual, elegant, conscious of her every motion, just like a show pony ought to be, but beneath the princess exterior is an eagerness to please. Generosity would make an excellent pet.
Also, for the Pinkie Pie fans: