It seems to be the following:
Twisted Treeline: Ability Power: 875 (or 725 with boots) From 1x Wooglet's Witchcap and 5x Deathfire Grasp
Summoners Rift: Ability Power: 936 (or 780 with boots) From 1x Rabadon's Deathcap and 5x Deathfire Grasp
Note that in the case of more than 1k mana, Seraph's Embrace yields more AP. Also note that Deathfire Grasp is interchangeable with any 120 AP item.
The above are without other AP effecting your build (i.e. runes/masteries/abilties).
My Kassadin build would then max to 1038.91.