I did some math to find out which AP items were the best if you wanted as much damage as possible. I only did the math for Void staff, Horizon Focus, Shadowflame and Rabadon (considering you already have Luden and sorcerer boots).
In the end Horizon Focus is always higher if you can proc its effect. So I’m wondering why every Lux player for example aren’t buying it second item.
But it’s just too hard to squeeze into builds or build at a relevant time frame. Assuming you finish liandry, rylai and demonic embrace you will most likely look for a magic pen item or survivability item or rabadons