Just know this, the best would be a speed reducing one+0 speed along with 31hp and 31 defense. You will copy all stats except Hp from the one you're agains (exception here is hidden power type) so there is no need for any specific speed reducing nature. For the rare events of ditto VS ditto you may also want 31 atkI would like a speed boosting nature (Ditto vs. Ditto would hopefully be very rare) just to be on the safe side. You can pick which one.