Zhukov said:
I hereby declare this question to be rather stupid.
It's like asking which is most important in a movie: image, sound, script or acting?
Correct answer: all of the above.
Same goes for games. I want a game to deliver a compelling narrative coupled with good gameplay in a interesting setting populated by well-crafted characters.
"Which is most important" be damned
The OP asked which is most important TO YOU. In other words, what is the make-or-break point of most games to you. Some people don't care much about story or graphics, but want the gameplay to be engaging. Others don't mind tedious gameplay, but rather just want a good story more than anything else. And there are others still who can forgive story or gameplay imperfections as long as the characters are entertaining.
Personally, I think I am in the latter two categories, for the most part. I have played and loved Final Fantasy XI and Grandia III, both which have EXTREMELY tedious and complicated battle systems, but FFXI is filled with great characters and deep stories and Grandia III also has a solid story going. So as long as I'm enjoying at least the characters or the story, I can stick around for quite a while.
I've never been able to stand games like Halo, though. It's not that I don't like FPS's--I am patently addicted to TF2 at the moment. It's that I don't like FPS's that just feel...generic. TF2 is very tight and the characters are hilariously entertaining. It is a unique and well-rounded experience. Halo, on the other hand, just feels like any other old alien shooter you'd see rusting in an arcade somewhere. Everybody is covered in those ridiculous suits, making everything a lumpy, samey eyesore.
Meanwhile, I know a lot of people who love Halo and simply could not forgive FFXI's contrived battle and leveling system enough to give the story a chance. So it's all a matter of taste when it comes to that sort of thing.