In all fairness, he did not ask about the most *overpowered* spell, he asked about the most powerful spell. In that matter it could be a toss up - the large amount of debuffing curse does (all elemental resists AND all stats) vs. the total negation of a skill.
Also, evil omen works in other ways - yes it negates your resist skill, but even if the next spell has nothing to do with that skill, such as a flamestrike, it still hurts a snot ton more. And it can be virtually cast on the run, having a bit less of a delay than curse.
Curse, however, can be devestating if the person hit with it is in a bad position. An extra second on the bandage CAN be the difference of life and death, particularly if more than one person is hitting them. Making a mage drop their shield if their str is a touch low can also be devestating.
Regarding wither - yes, it can be spammed, but honestly, unless you are literally stuck next to the spammer with no way to heal, you should be okay. If they are spamming with no delay, they dont have protection on and you can interupt the spell. If they have protection on, then they have a bit of a delay that should assist you in healing, not to mention they might well be working on low resists that make them very weak to your attack. I find a dexer to be the best bet against a wither spammer
All Kill is not a spell, and is certainly not the most powerful action - tamers are usually far too easy to kill to make that the case. Kill the tamer, problem solved in most cases.
I have also heard various arguments that strangle is high on this list, and it is equally devestating to dexers and mages alike, interupting the mage (and chiv healer) and dropping stam fast on the dexer. Stam drop = slower swing times, more misses, and less ability to move away for the heal to take effect. (note here - I say move away, I do not mean run 3 screens away. I dont consider that valid unless you are trying to avoid fighting altogether, such as being ganked. I tend to move in a wide, on screen circle around my opponent if I am waiting out a bandaid or disarm.)