As much horror as I read, it may surprise those who know me well to know that it isn't my answer to this question. I think I'd want to read thrillers. In thrillers we still have the scares and sometimes even a bit of horror, but there's almost always a conclusion to the bad. It offers hope. I need that glimmer of hope in my books from time to time or my reading would be awfully bleak!
I don't know if I could do it, that is, limit myself. A necessitating block or ban would slump me hard. It pains me to speculate about such questions of limitations.
If you could only read one genre for the rest of your life, what would it be?