I think the question stated in the OP is completely loaded and should not even be asked. It allows the reader to assume that whatever option they didn't choose can be assumed to be very very badly done (i.e. If I choose gameplay, I can assume that the story is totally shit/non-existent). Therefore, a lot of people can vote for gameplay simply from the argument that "If I wanted a good story, I'd read a book".
I think a fairer question would be; "If you can choose a game to have a the best gameplay or story ever created, while still scoring 80% on the other factor, which one would you take?"
I voted for story. Gameplay is the fun, while story is the added incentive. Fun by itself ,while being the most important factor, is not enough incentive for me to play games. Even a game with the best gameplay ever invented would not be on my personal list of favorite games (multiplayer games of over 64 players being the exception). If we're talking purely in terms of fun, then I can find a lot of other things I may enjoy more than games (I prefer sleeping and floor hockey over Wii Sports and Flower, for example). However, few of those activities can offer me the experience that a well told video game story can (I am perfectly willing to skip out on 6 hours of sleep for some more Dragon Age or Ratchet and Clank, but wouldn't even consider doing the same for Mario Bros Wii or LBP).