Can't personally speak for horrific, but it depends on the map, the spec, and the pt makeup. If I'm the only mage in the pt, maus 1-4 I generally prefer hex staff, then immo for 4-6. If there's two mages I always opt for one of each, if they use immo I'll use hex. Immo is by far the best if your pt can handle really big pulls, but you become incredibly useless when you get down to one or two mobs because it just doesn't proc enough.
I usually go fireball, clock, gale, shield with hex staff to get both DoT for proc, and also not dying from curse.
With immo I tend to go more defensive using heal, clock, gale, shield and play more like a mini tank letting the banish proc do all the work instead of worrying about actual damage output.
Again, I can't really say anything about horrific but if I were to speculate I'd say it's kind of an improved hex staff, which is how I'd personally use it.
In my VERY general opinion, horrific > hex > immo, up to m4. Immo > horrific > hex, from m4 upwards. That being said, each weapon has scenarios that it excels in more than the others.