For me, it's character.
Ad a writer, they are the first things that comes to my mind. If I sit down to find an idea for something new, it'll be a character that's going to come to mind first. The plot come later, usually from the characters past or present.
As a reader, I need good compelling characters more than a plot. Of course, something have to happen, but i don't need a lot. Characters is what's going to stay in my head post reading, and it's what I am going to remember in ten years if I reread the book.