Rarity is forever best Pony, and thus best Human.
That said, my deep love and passion for her is not in a romantic or sexual sense, but instead a respect and honorable fashion. Consider it looking up to a Queen etc.
But if we are talking pure head-cannon sexual and romantic appeal, then the list goes as follows.
1. Sunset Shimmer
2. Everybody else.
Tomboys do it for me, and frankly I see nothing tomboyish about Rainbow Dash. If anything she is a crybaby who is putting on a brave face and acting tough out of a mask to protect herself.