In Dusknoir, I say all three. But if you have to choose, it goes in this order:
1. 1/100 Dusknoir (Shadow Command is GREAT. His first attack can do 100 damage to the bench. His 2nd attack is an almost auto-win against Kingdra/AMU.)
2. Dusknoir DP
3. 17/100 Dusknoir (Not bad, but not good)
Try running all 4x in a Deck, including the Lv.X