I don't personally find any of the ponies attractive, but I'd have to say Fluttershy, Rarity, and Rainbow Dash all for different reasons. Fluttershy is the sweet natural beauty, Rarity is the glamorous sophisticate, and Rainbow is the sassy carefree daredevil. As far as the stallions go, I don't know if there's one I've seen yet that I really like. Maybe Soarin or Thunderlane?