Definitely gameplay, otherwise isn't it just an animated movie with annoying interactive bits? I don't care how good a games story or graphics are if the game-play sucks I won't bother, however if the game has awesome gameplay I can forgive the graphics and story for sucking. Case in point:
Dwarf Fortress, really crappy graphics (arguably part of the appeal), lacking a real story, but with incredibly deep and rewarding game-play, I have in installed on every computer that I use regularly.
Halo 3, crappy story, great gameplay it's still the only game me and my friends play when we get together.
Final Fantasy 10, Good looking, good story, can't stand the gameplay, I don't touch it.
Edit:
A caveat, if the art or the story is really inspired, then I will slog through the game to enrich my self, however this is only if the game is short, like and indy browser based title.