I think that content is probably the most important thing, I've seen great looking sites who had nothing interesting in them and never went back, however some of the sites I go back to over and over again, cause I find the information or images in them useful.
UI, Design, graphics and so on are basically there to support the content.
I have avoided useful sites because the design was bad, but if the content is really bad or unuseful or boring, no great design would save it.
Saying this I think that the boundary between content and design is not always clear and that it's basically a combination of all of the above.