I cast my vote for Stephen King. However, I think it's kind of a false dichotomy, as King and Barker are quite different authors, for all the fact that they are the only two horror authors that have ever really managed to frighten me. King's work takes place, for the most part, in the real world, in my opinion. Granted, it is the real world as filtered through his wonderfully bizarre imagination. Barker, on the other hand, seems to work more in a fantasy world, especially in his novels - and in fact his novels tend to be much more fantasy than horror, even while incorporating horrific plot points sometimes. It is Barker's short stories that come much closer to traditional horror, and even those are much more fantasy ridden than much of what King has written. Another difference in their writing is that King, despite his sometimes obscenity-ridden language, writes for a more general audience than Barker does - Barker's work is much more sexually explicit much more often than King's has ever been.
I do like both King's and Barker's writing. I find King to be more of a storyteller in the traditional sense. I also find his characters to be more believable, on the whole. This is not a criticism of Barker's talents - just an acknowledgment that he works differently than King does.