And the answer is....8
8
is the best pocket pair against aces. Why sevens don't come out to the same percentage I do not know, but it is 20.07% for eights and 20.05% for sevens.
For any 2 cards, either 87
or
or 67
or
is 22.87% against aces. And eejit is correct about the straight possibilities playing a factor here in both cases. 9 being the bottom end of a straight makes A the top end of that same straight. Whereas 6's cut off one possible straight.
I wonder why 8's have the advantage over 7's, is it because of the 2 pair possibilities? The ace kicker is going to win in all 2 pair situations with the 2 pair on the board being higher than 8s, and having 8s is better than 7s in that situation.