I think Kyon's sister would be dynamite in the sack. On the other hand, Chiyo would provide for you throughout your life what with being a genius and getting an awesome job. She didn't seem to age at all through her 3 years in high school either, so there's a chance that trend would continue later in life. This would be a plus, in case anyone is wondering.
One night stand - Kyon's sister.
Waifu - Chiyo.