While I would recommend playing through on EK your first time through, as it isn't too terribly difficult with the exception of a few boss fights and enemy encounters that are just poorly designed, and the difficulty achievements stack, you could play through on Poozer or Enforcer first. All the upgrades, constructs, collectibles, and xp you gain on your first playthrough will carry over into all the difficulties, so it could make your Emerald Knight playthrough a bit easier, if you don't mind going through twice. IF, however, you don't care about the EK achievement, and you just want to blow through ask quick as possible, Poozer would be the way to go, as it's nearly impossible to die, and you could get though the game in 4-5 hours.