What are the strongest summons?

[Taqwus]

A mage under a Dome of Corruption, casting spells during Astral Corruption, wielding the Banefire Sceptre and Magebane, wearing a Robe of the Archmage, Boots of the Planes, the Forbidden Light and Horror Harmonica should have a decent chance of seeing more horrors than he could ever want. No bets on his longevity 'tho.
Seriously, if you want a guaranteed-friendly horror, use Wish + Gift of Reason. Otherwise, well, you place your bets and take your chances with enslave/charm spells.