I see lots of people here putting THE STAND by Stephen King up as a favorite horror. With this in mind, I will recommend SWAN SONG by Robert R. Mccammon. Seriously, this is my absolute favorite book of all time. I know thats a hard thing to say, but this is the one that began my life of reading. It's not straight up horror, but it's more like the Stand. An end of the world book with horror themes. An amazingly vivid cast of characters and their stories get woven together perfectly.
Mccammon started out as more of a traditional horror writer with BAAL and They Thirst. But his later stuff like Gone South and Boys Life were just some of the best reading I could ever suggest to someone.