Speed for sure. If you can double your foes, you do twice the damage and have double the chance of activating any battle skills, so it's really more useful than strength or magic as long as you can do decent damage. Plus that's two chances of hitting the enemy, so skill isn't as needed if you're doubling and can do good damage.
Likewise, if you never get doubled, you won't need as much defence or resistance. And the same goes for Luck. If you don't get doubled, the enemy doesn't have double the critical chance.
So my ordering is: Speed > Skill > Strength / Magic > Defence / Resistance > Luck