Hex Decimus:
With a Power Item you are guaranteed one stat to be passed down, but then Destiny Knot will only pass down 4 random IVs in stead of 5.
Because of this Couple B and D are equally good, but Couple B has the benefit of letting you pass down natures with everstone. The chance of a 6x31 offspring with those parents is 1/32.
The next best is couple C, since then you only need to be lucky and have 4 random 31 IV-stats be passed down, in stead of 5 which is the case with couple A.
In totalt, from best to worst with regards to only IVs:
B=D > C > A
With a Power Item you are guaranteed one stat to be passed down, but then Destiny Knot will only pass down 4 random IVs in stead of 5.
Because of this Couple B and D are equally good, but Couple B has the benefit of letting you pass down natures with everstone. The chance of a 6x31 offspring with those parents is 1/32.
The next best is couple C, since then you only need to be lucky and have 4 random 31 IV-stats be passed down, in stead of 5 which is the case with couple A.
In totalt, from best to worst with regards to only IVs:
B=D > C > A