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In order, my top 5 list:
Rabadon's Deathcap - Must have on any AP carry late game, and highest AP bonus even considering Mejai's (140 +30% of all AP = minimum 182 AP from just Rab).
Sorc's Shoes - Early game MPen and move speed, two major things that mages need.
Abyssal Scepter - AP, MR and MR shred. Fantastic combination, and helps your teammates.
Athene's Chalice - Situationally more useful than AAS, as most AP goes mid and this gives more AP and MR
Hextech Sweeper - Only on Dominion, but still a fantastic item, particularly if you use skill shots. HP, CDR, AP