I gotta go with Cloud of Darkness.

It can instantly 1 shot any being that isn't 100% neutral. The heroes of light had to absorb the heroes of darkness(thus making them neutral) in order to defeat it. I say defeat since CoD can't be permanently killed.

Now factoring in that no other FF hero or villain has a dark or light doppelgänger to absorb; CoD should get an instant win on any of them.