I think defense is the best. Let me explain: Attk and Strength take 3 or 4 levels to raise a combat level. Defense takes 4 or 5. Now defense is a combat stat just like attack and strength, and theoretically, a 90 def cancels out a 90 attk to make the high attack mediocre. However, the combat level remains lower and allows more room to raise attack and strength levels on the player with high defense. Remember, this is just a theory, it is not tested.