I vote for Hermione - Luna is too dreamy to be a good fit for someone so goal-oriented. She drifts, and take it from someone who knows:
1 The Messies drive Neats insane.
2 Procrastinators are a continual irritant for those who "just do it."
3 She never fights back (witness all the hazing incidents in Ravenclaw) which makes her someone to protect, not respect
4 She never gives a straight answer, and fuzzy information gets people killed.
Hermione is strong, ambitious, and adaptable. She's also disciplined and she stands up for herself.