Originally Posted by Chthon

If you're running a CoP build properly (or, in the case of a cryway team build, even a bad build badly), your target is going to die so rapidly that the actual effect of your proc hex is going to be minimal, no matter what it does. With that in mind, the criteria for picking your proc hex should be energy cost and casting speed. Fragility and Mind Wrack lead the pack as the only 5e, 1sec cast options. At least for standalone builds, I prefer fragility for the synergy with Finish Him (if the target survives long enough), which in turn synergizes with AP. Mind Wrack is second. If you're willing to come up to 10e, my third choice is Web of Disruption -- 1/4 cast, interrupt utility, and no attribute points required to work at full power. Frustration is maybe fourth or fifth.

In any event, Ether Nightmare is the unquestionably worst proc hex for CoP in common usage. It's got a three second cast. A. Three. Second. Cast. Just to add insult to injury, it's also 15e; the supposed benefit -- AoE hexing that leaves you some pre-hexed targets when you finish with your current target -- is often negated by the fact that CoP kills everything in the same AoE radius; and the effect -- degen -- is only effective if it has time to run its course, exactly the opposite of what you're trying to accomplish with CoP's quick direct damage.